Look no further,
look here. Dealextreme is not UK based, but they ship worldwide without additional shipping costs and they're reliable. Shipping usually takes about 2 weeks, which should be bearable.
I'll advice you however to take the Supercard SD, since it's much more flexible (it's also on the page).
And you'd better be quick, wherever you might buy it, since GBA accesoiries and carts are quickly running out of stock everywhere and the GBA itself (although it's still highly playable and I still like it) seems to be abandoned in favour of the upcoming DSi.
Thanks for the info Squirrel61.
Do you have a Supercard SD? Have you experienced the slowdown problem I've read about?