thing is, criteria is often a first from UK universities, The only expection being oxbridg, sometimes durham, manchester, warwick are exemptions but not always. here's another thing, you have, very often, the a-levels to get into durham or manchester, you have the alevels to get in oxbridge...
so why oxbridge is the only exemption? when a student who gets a first that they got into the uni with a ABB is preffered over a low 2:1 from Durham, considering a-levels everyone is in the same boat, universtiies are largely varied, u do a gr course at one uni, the exam paper atanother uni is on a differnt level potentially, just cause you both did a 'gr' course doesn't put the two candiates the same, and moreso then the papers differing for similar named modules at universities, the studies at a university are scaled to the entry requirements right? so it makes little sense to compare a 2:1 from a uni with A*A*A requiremetns to a uni with ABB entry...it is insisting that the student who got the stronger a-levels would not have got a first if they went to the same uni as the ABB candiaite- who'd bet on that? very few, I'd imagine.
So yes, with the exception of perhaps the top 5 or so UK unis, which do attract firsts from top unis aound the world, i'd say you're good enough. Also a degree is largely varied, some modules you like, some you don't, some students could have selected modules that were easier that year, judged by a record of the past historyof papers etc or somethintg like that.