There's really only a couple of options if you're going to buy one. The cheapest and only synthetic model is the Snugpak underblanket designed by our very own Wanderinstar, Lakeland and DD stock them now I think.
The other option is a down version, obviously a lot lighter and packs down smaller but you pay for it. You can get a 3/4 length version from Unsponsored (look in the classifieds) which will see you good through a British winter if you pair it with your sleeping bag. Another option is to buy from across the pond, have a look on hammockforums.net for some info and a list of the guys who produce them. I've got the Speer Snugfit which cost a bob or two and I've got one of Phils 3/4 bags, I use the 3/4 for most of the year but I'll be switching to the Snugfit for my next trips now.
For your budget go for the Snugpak, it packs to a similar size to a 3 season synthetic bag and comes in just over 1kg I think. I used one for a couple of years and was really happy with it, I only upgraded to the Snugfit and 3/4 to shave a few pounds.