If you don't need a full 88keys (lets face it, you're not going to get that for £100!), I would recommend the CME UF5. It's around your budget, and although build quality seems to be a little spotty, this is an excellent keyboard with lots of controlability.
http://www.dolphinmusic.co.uk/page/s...roduct_id/5743
There are also a few smaller options by Edirol and M-Audio that fit the bill. Personally, I think the action and feel is better on the UF5, though it does take up quite a bit of room.