Planning the wiring now, and getting it done in the first fix, is a good idea. As you've realised, it's much cheaper getting this done when the walls and ceiling are open rather than after the surfaces have been finished.
Deciding what cables, and where they'll terminate, and then making sure they're pulled correctly (to a proper plan rather than leaving it to a Spark who may or may not get it right), and labelled (yep, they forget to do that too), and then not subsequently damaged by following trades nailing or drilling in to walls and ceilings can be something of a challenge. It's not insurmountable. I do it in lots of new build and major refurb' projects.
Have a look at this layout plan by Dolby. People often refer to rear speakers in a 5.1 layout, but they're not rear at all. Those speakers are to the sides of the main listening position. Here's the
link to the picture.
Where you're planning to have in-ceilings to the sides (5.1) and rear (7.1) then the type and positioning of the speaker needs some thought. You'll need to have a very clear idea of where the seating will go in the room. In fact, the positioning of everything - the gear, the speakers, the sub, even you on your seating, can be planned before the footings are dug, and in this way it's possible to avoid a lot of the sonic problems that get in the way of really good stereo and multichannel performance.
Coming back to specific bits of gear; subwoofers. Your main speakers are rated to get down to 53Hz. Quite whether that's 53Hz before the output starts rolling off or at the quieter -3dB point, or the much quieter -6dB point isn't clear from the QA web site. -3dB is a typical quoted figure, but some makers use -6dB when they're trying to flatter a products performance. In room, your speakers will sound like they dig a little deeper as they interact with the room itself. Closeness to the rear and side walls can load the bass with a little artificial boost. Get too close though and the boost turns in to boom and bass overhang.
Bringing this back to the QA subs, they're not really big enough (driver size and cabinet volume) or powerful enough (amplifier wattage) to make much of a useful contribution to your £800+ floor-stander main speakers. The QA subs are designed to partner their smaller bookshelf speakers which, due to cabinet size, are far more limited in bass performance.
QA's biggest sub gets down to 35Hz, which really isn't going to make much of a useful contribution alongside your main speakers. As an alternative to their £300 sub, you should consider something with a larger driver (12" vs QA's 8") and more power (250-400W vs QA's 150W). My first port of call in such situations is BK Electric. It's a brand that sells direct to the general public, so you're making a £100-£250 saving straight away because there's no dealer margin sitting on top of the factory gate price. This gives you a significant performance boost for your money compared to retail brands.
Given your current speakers, and the fact that this will be a dual music/AV system, there's no point messing around discussing small-driver/low-powered subs. Go straight for the Monolith 12" 300W bass ported sub. It digs deep enough to give even some organ pieces an impressive presence, and it has enough power to make movies really rock.
BK subs have a neat trick up their sleeve too. It's a dual connection. Alongside the usual low level AV connection from a home cinema receiver's subwoofer out socket, the BK subs also have a connection called high level which is for the more powerful speaker level connection from an amplifier or receiver. Connecting in this way supplements the bass performance of the main speakers only. That means when you're listening just to music, you have the benefit of the sub working with the main speakers so it fills in the missing bottom end. In effect, it's like upgrading your main speakers to something far larger, but without the cost or the size or the additional power required to make that change.
In your case then, this would connect to the Rega Brio speaker sockets in parallel with the existing wiring to the Concept 40s. The natural questions most Hi-Fi enthusiasts then ask are 1) won't this pull power away from the main speakers, and 2) will it change the synergy between the amp and the main speakers? Happily, the answer to both those question is No.
The high level connection to the sub is at such high resistance that it draws virtually no current, so it's not seen by the amp as a speaker load. As far as the amp is concerned, the sub is invisible as far as power consumption goes. Also, because the main speaker signal isn't interrupted, it doesn't change the character of the sound of the amp and main speakers together.
There are other brands of subs that use high level connections in a different way. They connect in-line between the amp and mains speaker; daisy-chain fashion. The speaker signal goes through the sub first, and with some of them, it is actively filtered to cut away a portion of the sound that's then fed to the sub. There are pros and cons with this. On the plus side, it can reduce the load on the amp which means the amp has an easier time driving the reduced frequency range going to the main speakers. That can be a good thing, a bit like running your speakers with a marginally more powerful amp. The negatives are that it's introducing some extra circuitry sitting directly between the amp and the main speakers that wasn't present before. Some say they can't hear a difference. Others, that they can.
Coming back to the BK, its high level connection doesn't get in the way, but it doesn't take some of the heavy lifting away from the Rega. If you're following so far, I'll finish off here by telling you how the high level and low level (AV) connections work together on the BK subs.
The BK sub can accept both signals at the same time. On the rear panel, there are two sets of adjustments. One set is for blending the sub with the main speakers. This is the high level crossover. The second set of controls are for the low level (AV) connection. Most of the settings for this are actually handled by the AV receiver.
When playing just music (Rega amp on, but AV receiver not on) then the sub is getting it's signal only from the high level sockets connected to the Rega amp. When it's time to play movies, it's the Rega amp that's still driving the main speakers. It gets its signal from the AV receiver via an unused line input on the Brio. Your Brio connections then are CD (the Marantz) plus any other dedicated stereo only sources (in my case a turntable and the stereo line out from a very good CD/DVD player), and a stereo connection that uses the AV receiver as a source to the Brio. It could be any of the line inputs; Line 1, Tuner or Tape, they all do the same thing. The AV receiver connects to the TV, DVD/Blu-ray player, Roku/Amazon/Google/Apple streaming device and passes the front channel signals to the Brio while handling power to the centre and surround speakers itself. There's a low level subwoofer connection direct to the BK sub. This carries low bass from the film low frequency effects channel (LFE - tyrannosaurs stomping about, the deep bass bits of explosions etc) plus any part of the standard audio spectrum that the centre speaker and surround speakers can't handle. The BK sub combines and plays all this as if it were one signal. That's pretty clever. You get the best out of your main speakers for music, and you get full effects for movies, with nothing more complicated than two bits of wire and some great thinking from BK.
To complete the picture, the AV receiver has to be one with a set of Pre-Outs for the front L&R channels. Marantz SR6013 @ £950, Marantz SR5013 £tba, Denon AVR-X3500H £600, Pioneer VSX-LX503 £800, Yamaha RX-A880 @ £750 all fit the bill. It's a little under the radar, but the Marantz slimline receiver - SR1609 @ £449 - also has front Pre-Out connections. Being a slimline unit, it doesn't pack quite the punch of it's bigger siblings, but with the Brio driving the front channel speakers it might make a nice companion for your intended system. It's 4K UHD TV compatible, and does ATMOS if you desire too.