Personaly for your budget i would look at an all in one system,unless you plan on upgrading seperates ie:speakers at some point.
Onkyo are a very good make,as are Sony,Yamaha etc...it realy comes down to what fits your requiremants and budget best....there are a few well known retailers that do some great deals,richer sounds is one,but a seperate system will be a little more than£300.
£300 for a package you will get tbh crap. It will be alright but you'll find yourself quickly wanting to upgrade and it will be depressing especially when testing out speakers before buying and seeing what better speakers could do for you.
My opinion would be get a receiver and 2 front speakers and work your way up.
You will get a better sound having a good front sound stage that does your main sounds than a lower quality surround.
My suggestion is ; last years Dennon 1912 which can be found for about £250 but is scarce now. And some Q acoustics 2010 or 2020
It may take you Over budget but be better saving maybe little longer and getting them. After that your looking at £250-300 for significantly better speakers.
Since you have a Wii a receiver with upconversion would be useful as you would only need one HDMI from the receiver to the TV. Unfortunately unless you take Nevaborn's advice you do not have the budget for a receiver with this feature. Hence you would need to run a HDMI and component cable to the TV.
If it has to be full surround from the start then look at these:
The Onkyo has the best receiver in the package but costs the most. The Yamaha is the cheapest but only has a passive sub. The Pioneer has a similar level receiver to the Yamaha but with an active sub. For me the Onkyo is probably the best option out of the 3.
sparky you wouldn't buy a car without test driving it first,the same should be said for any AV aquipment.
find a good local retailer,sevenoakes,richer sounds,superfi etc and go see them.you can get an idea of what you want from forums then you can get the store to demonstrate that aquipment so you can compare and choose which you prefere and you will form an opinion.....'av fun