*cracks knuckles*
Here we go, lads!
First off let me say that Onkyo are really shaking things up, releasing brilliant AV reciever after another. They are great prices too. The TX-SR505 is a really solid reciever and it's only downside isn't an insane number of inputs but more than enough for the mainstream user.
It doesn't support the new low-less audio streams but that is fine since the Xbox 360 doesn't support them anyhow (and neither does digital optical).
Normally, I'd recommend Cambridge Audio for bang/buck but Onkyo have really upped the game recently and even though I'm heavily biased towards the UK brand, I'd have to concede and buy Onkyo if I were on a budget.
So, speakers.
Finding a great 5.1 set for £200 won't be easy, but you will be able to get some decent drivers. My personal preference would be Mordaunt Short, but a full system is out of your price-range. I have, however i remembered a nice little speaker package by Tannoy:
TANNOY EFX5.1 at Richer Sounds - HI-FI Separates, Home Cinema, Speakers, MP3 DVD Portables, Plasma LCD, etc.
A great set for those at the "budget" end of the scale (meaning low-end, beginner audiophile kit). That would go really well with the Onkyo. If the £250 is a bit much, try looking at the Tannoy FX5.1 instead - only real differences is a slightly less amazing centre and a different subwoofer.
As for wireless rear speakers -- I wouldn't suggest them in a home cinema. They are fine with the multimedia end of the market (i.e. Logitech Z-5450) but for home cinema they are not very popular. I myself run my speaker wires behind the plasterboard and have interconnects on the walls themselves but that is only really possible if you are building the room for that specific purpose. Try running them under the carpet.