How should foreign consoles be powered in Britain?

You shouldn't need anything additional to power your N64 in Britain. I've found a few sources to back this up https://forums.the-elite.net/index.php?topic=19934.0 and https://forum.speeddemosarchive.com/post/running_ntsc_consoles_in_europe.html (you can find more by searching "EU N64 Power".

Essentially, they are powered by an external PSU that converts the AC voltage to a D.C. voltage for the console. Just make sure the PSU is rated for the socket you're using (should be rated for both EU and NA standard power).