This is a US website but it might give you a few ideas:
http://www.chiff.com/home_life/shop/most_popul ar_toys_list.htm
It could be worth looking in your local Maplin Electronics store. They often have excellent value toys (which are usually educational but always great fun), suitable for the age range of the youngsters you're buying for. In particular, keep an eye out for easy assembly kits which let the user build things like radios and then take them apart to make something else. (I've bought those, on several occasions, for youngsters of around 10 to 12 years and they've always been very popular).
Chris