There are (at least) two ways to join this community if you come from other instances:
Go to the search of your instance (there’s an icon on the top-right), search for ‘cartalkuk' (you might want to limit your search to communities) and from there click on the “car talk UK” link.
Manually copy-paste the community name to your address bar. You need to add c/[email protected] to the address of your instance. E.g. if you are on lemmy.ml: https://lemmy.ml/c/[email protected]