I'm sorry, are you high?
For English Law, nowhere in the world is better than Oxbridge.If you were to ask what UCL is better than Oxbridge for, I'd say arguably some postgrad things, notably neuroscience, but apart from that, nothing.
People get offers from better places than others (strictly prestige wise) because places have different entry standards/look for different qualities. I knew of someone who was rejected from everywhere except Oxford for PPE, but I'm pretty sure you wouldn't say York was better than Oxford for PPE.
In short, the reason they have an offer from Oxford is probably becuase they had a good interview, and that made up for some lacking in their application (perhaps LNAT, Oxford is known to accept people with relatively low LNATs, whilst UCL does not.)
Total opinion though, and I probably could be said to be extremely biased.
Should be noted also, for humanities/social sciences, Oxbridge are usually top 3 in the world.
UCL, whilst undoubtedly a fine institution, is not.