UCL with Durham very close behind. Then Warwick and Manchester.
Culture / environment is entirely subjective. If you're from London, UCL might not be for you, if you're from the Home counties, Durham is pretty much stacked with people from London, Surrey and Hertfordshire. Durham is Diet Cambridge in many respects, with the nice buildings, the bailey colleges etc.
Manchester will have the nightlife. Warwick will exist.
Law firms internationally - UCL is always claiming to be the big international university, and in many ways the way they sell research, there's some truth to this.
As with everything with law, you gotta have a first from a top university. Is a 1st from Durham or UC better than one from Leicester? Of course.
This question comes up all the time - I would weigh up the chaos and expense of central London against the perennial Durham student housing crisis. It's not impossible you'll end up commuting from Newcastle or Gateshead. If you want to say you rowed for your college - then Durham. If you want easy access to pro bono and to have lunch at Sketch, then UCL. If you want mad nightclubs and a music scene, then Manchester. If you want to go to university in Leamington Spa, then Warwick.