UCL is a very good uni, but the mech eng isn't the best, as azamc said
the engineering dept has a brand new building, which opened about 2 years ago, but obviously there's still a lot of old ones.
i had a few mates studying mech eng at ucl for both undergrad and postgrad, and i think it's mostly very good, but obviously like with anything else, you might get a shoddy lecturer.
no idea about the rankings, but whilst i do know that UCL isn't the best for mechanical engineering, it still is a very good university
having studied at UCL, i'm biased, but you certainly won't regret it. what are your other choices?