Cardiff or London.. Wow prepare yourself to make a lot of Muslim friends from all imaginable backgrounds?Cardiff has a smaller Muslim population but it's enough. There is nothing to worry about.
You can join Islamic societies and whatnot, chances are you will acctually meet someone from your country if not city/village lol talking from own and friends' experience.
Anyways that's enough about Muslims ,other people are just as great and understandable especially at further education level. I mean there will obviously be a few douchebags who will make fun of you but it's not like you would want to hang out with them anyways, that's the nature of humans some like you more and some less.
Hope it goes all well for you and you enjoy your time in the UK