The brief is still pretty vague. But reading between the lines it seems like you are at the build phase so you have the opportunity to prewire and plan for the install. It might also mean that you haven't yet bought a TV. Also, your comment about keeping things simple could be interpreted a couple of different ways; either "
inexpensive" or "
very user/family friendly". The second of those choices could put several hundreds to several thousands of pounds on to the budget.
If you just want a "get you going" solution then have a look at a 5.1 all-in-one home cinema kit. Budget from £300 up for something with a CD/DVD/Blu-ray head unit with iPod input and a couple of additional inputs for TV audio and a games console. You can buy this sort of thing from Currys or Amazon or the local electrical retailer. The speakers have keyhole slots for hanging off screw fixings in the walls. The speaker cables will need extending to cope with your room size, so if you go down this route you should by the kit fist and have a good look at it before you start the building work.
From approx £500 you start to get in to the realms of a proper 5.1 separates system (a BD player + amp + speaker kit). This is still DIY install. But here the advantages are audio quality, greater flexibility and better user features (e.g. iPhone/Android control).
This isn't for everyone, but if you want to create a real
Wow! space then from £1000+ you can start to talk to firms like mine who will do a planned install.
The benefits are a properly thought-out system design: We take care of all those "benefit of hindsight" things are often overlooked by the DIY'er. A professional install also gets the most out of your gear from the word Go and shows you cost-effective upgrades that means your family and friends gets the most from your investment in the space. I'm based in Middlewich, but there are probably firms even closer to you as well.