There are two methods for playing Import Mega/Sega CDs on a UK Mega CD - one rather fiddlier than the other.

The easiest method is to get hold of a Datel CDX Cartridge which you pop in the cartridge slot before loading the CD up. However, there are a few revisions of the cartridge and it can be hard to find the later versions that work with the Mega CD 2. However, if you have a Mega CD 1, this would be your best bet.

For the second method, you'll need a PC with a CD Writer. Firstly, you need to create an ISO image of the import game you're trying to play and use a program called ConvCD to change the region of the game and then re-burn the game to a fresh disc and you'll then have a European version of the game! Again, this method doesn't work for all games and requires extra effort but it can be worth it if you want to play a game not released over here. Be sure to read the documentation for the ConvCD tool!