Northumbria work in UCAS points of 120 which is BBB or equivalent. Cardiff can offer AAB if you’re doing an EPQ and get an A or above in it. But Cardiff are often in Clearing and do sometimes offer ABB then. It varies by year of course.
Northumbria isn’t a bad course, their Law School is quite good and it has a decent reputation for the Bar Course (realise this is not the undergrad, so apples v pears). But, if the OP is going back to Greece I’m not sure their degree would be of much use!
I visited Cardiff and thought it seemed good for law. It didn’t make my shortlist in the end. They offered some interesting optional modules and some good links with the local legal community, as well as some very strong pro bono opportunities (some of the best I saw actually). Again however, not sure it’ll help anyone returning to Greece to work.
Realistically OP needs a university with good international standing like LSE, but that’s a whole different league.