Playing imports on an N64 uses a similar to that of a SNES – your best bet is to try and pick up an import adaptor on Ebay and use an PAL Cartridge to piggy-back onto the back of the adaptor. Results can vary depending on the import game you’re trying to play and the PAL game you use to boot from. Personally I use WWF Warzone as that’s about all it’s good for! You will need an adaptor of some kind though as an Import cart won’t quite fit into a UK N64 slot – not without some filing down anyway!
Some of the later games will still refuse to boot using this method so ideally, try and get an import adaptor that allows you to enter Action Replay codes. Personally I recommend trying to get a Passport Plus III (tends to sell for £25 on Ebay unfortunately) as this allows you to enter codes and also comes with a lot of useful codes built-in.
If you can’t find the Action Replay Boot code on the Internet for the game you want, try using a Boot code from another game as that will often work. It’ll take a little trial and error but it can be worth it to play games like ‘Hey You Pikachu!’ and er, ‘Mia Hamm Soccer’.