You can get all the great coffee you need here, there are hundreds of independent roasters and you can't walk down the high street without bumping into a barrista. Coffee wankery is at least as advanced here as in the states.
https://ravecoffee.co.uk/
Don't stuff your luggage with coffee, it will only get old and stale before you can drink it. Far better to bring grits, those are difficult to find here.
Jimbo's right, great coffee is easy. That Rave Coffee looks great, but only once a month?!?! I'd never live. I use
https://www.pactcoffee.com/ -- they make it easy to get as much or as little as you want. We go through approximately three bags a week, and I haven't had one yet that wasn't delightful.
Have also gotten Union, ThreeSixty and Orang-utan brands via Ocado, just to have back-up coffee around (especially important in this flat over xmas or a bank holiday weekend), and all of those are excellent quality.