Eibach Sportlines are the most popular and cost effective drop you can give your GP. You don't sacrifice too much ride quality (none IMO), and you'll be fine with those pesky speed bumps on British roads. They will give you a decent drop, without going stupidly low...but enough to complement the smooth lines on the GP. I run them, have done for a few months now and by far the best thing i did to mine to date. Plenty of others use them also, the list of names would go on and on and on
There are some second hand ones going in the classifieds section currently, or you can get them brand new off of eBay for £160-180 ish.
Eibachs Sportlines, there are two kits Sportline/Pro-kit. Go with Sportlines, slightly bigger drop. I forget the colour difference off the top of my head, ones red, the other is black (box is the same). Think the sports are red, pro is black...so think its red your looking for