300 pounds is not a whole lot for 7 speakers and a subwoofer, you might want to consider something like a home theater in a box. I would search on amazon uk.
What speakers are you currently using? For 300 pounds I would probably look to use that to replace the front 2 speakers.
for my front speakers ive got 2 misson speakers and for the side speakers i have 2 Whatedale and for the center speaker i have a sony and for the back 2 i have i think thay are Whatedale as well but different model from side speakers and the sub is a sony that cos me £8 on ebay.co.uk lol