TBH, I'd avoid floorstanders with your budget. There just isn't enough money in a £150 pair of floorstanders for the manufacturer to make a strong and well braced cabinet. So what you gain in a little extra bass depth from the bigger box is outweighed by the loss in precision.
If you're going to buy in stages (which I do thing is a good idea) then you need to plan ahead. The front three speakers should be from a matching brand. If it's going to be a while before adding a centre, then buying bargain front L&R speakers now when there's no matching centre in the future will leave you a bit stuck. The centre really is the most important speaker for movies and gaming. Most of what you hear comes from the centre. The front L&R speakers are really just adding music and effects and a little to help with the steering of panning across the front.
The way I see it, your budget breaks down like this: £100 for an AV Receiver. A centre is a minimum of £99. That leaves less than £100 for front L&R speakers and sundries (cables, interconnects, speaker stands etc). The maths doesn't add up with making some large compromises on performance. What you'll end up with is either a system that doesn't sound that good, or that you have to upgrade sooner rather than later.
If it was me, I'd go with stereo speakers and an AV Receiver. When the AV Receiver is configured in 2.0 mode then the front stereo pair will handle the centre speaker duties, so you want some decent speakers for that.
I'd look at Dali Zensor 1 speakers (£199). Add a £100 AV Receiver. Scrape together an extra £30 for some speaker stands and ask Richers to chuck in the speaker cable.
If you're still wedded to the idea of floorstanders then have a listen to the Mission 33i (clearance @ £149). Be aware though, the Missions only go down to 48Hz. The much smaller Dali's nearly match that (53Hz) and will sound more accomplished in other areas.
Secondhand speakers is always an option, but because of age, choice, distance and a whole raft of other complications then I think s/h at this budget should only be entertained if you really know what you're buying.