Thanks guys just toying with the idea at the moment so doing a bit of research. Budget would probably be around £400, and the bluray player isn't that important as I store blurays on HDD played through a small HTPC. So what does £400 in amp and speakers get me? Any "smart" features are irrelevant with the HTPC, but must be 5.1 speakers.
Features-wise the speakers will be 5.1, so a centre plus 4 surround speakers and a subwoofer. The AV receiver will have inputs for all the commonly used connector types (analogue stereo, optical, digital coax, HDMI) and with adapter cables you'll be able to also connect other peripherals such as tablets and phones: This will cover you for everything from old legacy AV gear through to the latest AV sources.
Once your gear is connected, then the next feature-sets you're interested in is power and decoding.
Those all-in-ones with the built-in disc player boast some pretty impressive looking wattage figures. When you compare that against a typical AV Receiver then it looks like the slimline disc player holds all the aces. However, the figures are a smoke and mirrors trick. How they're measured and the way they then derive a result is a magnificent work of fiction. If you want the details then I can explain, but the bottom line is that you can pretty much take the claimed total wattage figure and divide by 10. You then need to further divide it by the number of speakers being driven (5 usually)
AV Receivers deliver a greater amount of real power. They're still subject to some optimistic calculations in some of the specs, but if you read the "Rated Output Power (20Hz-20kHz, 2ch driven)" figure then that's a pretty accurate indicator. Running 5 channels in the same way will produce a drop in power because it's being spread across more speakers, but you're still looking getting about 60-70% of the claimed 2ch power so long as it was measured at a low THD. So, an AV receiver delivers far greater levels of real world useable power. That's good news for dynamics which is the ability of the receiver to cope with big instantaneous surges in volume for things like explosions or peaks in music. It means that the receiver is less likely to distort when the going gets tough. The all-in-one kit will run out of steam and start to sound harsh and unpleasant at much lower volume levels.
That's the power differences sorted out. Now let's look at what happens with the decoding.
First, your typical AV Receiver: It has everything. At the top end then you have DTS-MA and Dolby TrueHD both covered. It will also handle PCM multichannel for when you're connecting devices such as PS4. There are times when allowing the source device to decode audio is the best option. Next, legacy audio is covered with DD and DTS, and you have Dolby ProLogic II (DPLII) for turning basic stereo signals in to surround. On top of this you have further modes optimised for music and for gaming. You'll probably have these in both Dolby ProLogic and DTS Neo versions. This means that pretty much anything you throw at the amp can be handled and the amp will either do so automatically or give you the option of choosing between some very effective processing algorithms.
Behind the scenes you'll often find that AV receivers offer greater precision in the way they set up, and have a wider array of controls for customising the way the system deals with signals. For example, TV audio can be set to be decoded as DLP II Movie by default unless the signal from the TV audio out is Dolby Digital 5.1
Compare and contrast this with the options in an all-in-one. At £400 you're likely to be looking at a system with a couple of inputs as well as the disc tray as a source. While the system may handle HD audio and DD/DTS and maybe even ProLogic, you might be surprised to find there's some holes and omissions in the pattern. For example, the external HDMI inputs don't allow HD audio decoding or even basic ProLogic.
Next there's the set-up controls. It's common to find that the settings for speaker distances are much coarser, so it's more difficult to get precise effects placement.
I've only scratched the surface, but I hope it's enough for you to see that you are dealing with two very different types of system.