hmm, judging from the rankings from Shanghai Jiao Tong (which I hear are supposed to have the most well regarded rankings system), Univ College London is number 4 in Europe, wheras they rank Queen Mary Univ London as 83-123! This was for 2005 though, I can't seem to find the upto date ones.
Times Good University guide ranks UCL as number 6 and Queen Mary as 42, so it seems Queens is quite a bit of UCL yet again, however this a general ranking of course, not physics dept specific.
I have no experience with the Queen Mary physics dept, but I do think it's supposed to be quite small, but has a good reputation for string theory (Think that's the place Michael Green was actually at during the conception of string theory), so it probably has a string theory dept that has a relativley better reputation than rest of the uni.
If you're interested in plasma physics I would just check the research area of both the UCL and Queens dept website, and see which seems the best from there...