That is, if you can find a Freeloader in the UK. Your best bet would be to order that online.
To add on more to what Latios said, here's a little explanation.
Most consoles of today use discs. Those discs end up being region coded, and said consoles end up being locked to use only their specific region. In other words, you cannot use a Japanese GameCube game on an American GameCube or a European GameCube. It will only work with the Japanese GameCube. Same thing applies for the others.
The Game Boy Advance and Nintendo DS, however, are different. They use cartridges/cards that are not region locked. Meaning you can play a Japanese GBA game on either a US GBA or NDS. Or you can also use a US DS game on a European DS. Neither handhelds are region locked, so you can import without worry.
Yes, we do have freeloaders in the UK. If we've just imported a game, and we find out you need a freeloader, not many people would waste more time importing one now, would they?
Like said, use a Freeloader. If you got the disc for Jirachi, you don't need to get anything to make the game work with a european GBA.