If you've got a 42 incher, don't skimp on your audio.
Take a look at whathifi's top list for amps:
http://whathifi.com/Reviews/Home-cinema-Reviews/
http://whathifi.com/Review/Onkyo-TX-SR505/ If you don't want to break the bank. Once you get into the audiophile and videophile world there is no going back. You can get basic models like that right up to £5k+. That said, in my opinion once you start paying more than the TV you should consider your priorities (and as you know, TV's can go WELL above £5k!). I don't know how much you spent on your screen, froogle says from 500-800 so assuming about £700?
http://www.onkyousa.com/images/produ...R550B_rear.jpg That is the back of a typical amp - as it happens, it's the ONKYO i posted - so a little bit into how it works. Essentially, everything that's not the TV or the Speakers goes OUT from the device and IN to the amp. The TV has Video going IN and audio going OUT to the amp. The Speakers will take the amp's audio out. The amplifier/reciever mediates between all your devices :) That ONKYO also comes with a radio antenna i believe so you can listen to AM/FM radio if you want. It's rated to 7.1 (one sub, front L,R, surround L,R, back L,R and a front centre) which is about as much as you can get (some people have weird 7.2 set ups but eh). £250 is an absolute bargain for an amp like that - and it also comes in black if you're so inclined (i am)!
The point of paying good money for your amp and speakers is that they will last you through all your TVs, whereas you might pay loads for a TV that you want to replace in 5 years.
Speaker wise, there are no end of 5.1 set ups - which is what you'll be looking for for basic surround sound. If that's pushing your budget (don't waste your money on a cheap set up, you'll only regret having to replace the mediocre sound later) then a decent 2.1 will be perfectly adequate for everything.
£500 will get you something in the region of:
http://whathifi.com/Review/Q-Acoustics-1010i-51/ and of course you can go up from there. Cheaper option with excellent reviews is:
http://whathifi.com/Review/Jamo-A102HCS5/.
Cable wise, don't be conned into buying expensive cables, numerous forums have shown that in a lot of cases it doesn't make jack all difference unless you're in a restaurant with neon lights everywhere. Cheap ones will be fine, but it will be the build quality rather than the cables not being up for it that will screw you over.