If you want proper kit
Cotswold camping in Reading stock a good range of kit and aren't too far from you. Alternatively
Field and Trek in Guildford are well stocked but a bit smaller.
I use a
one of these which has done me well in horrible winter weather and isn't too bad up to relatively warm temperatures, in a compression sack it's about a cubic foot. I've also got a much thinner
2 season which pack to half the size and is fine for indoor use but you need to leave a T-shirt on for outdoor use in all but the warmest summers.
Down will always pack smaller than fibre, but tends to be more expensive. Compression sacks will make it a lot smaller bundle to carry but usually make it a shorter cylinder rather narrower cylinder if you see what I mean.
Milletts and Blacks will stock a basic range too.