I remember one time having to do a sleep study at St Thomas's Hospital - 2011 I think. My room overlooked Westminster Bridge and during the day it was the usual bagpipe player, Chinese wire bender, and Eastern Europeans selling chestnuts and dodging the local police. At about 7 in the evening it completely changed and you had Eastern European gangs running betting scams on unsuspecting tourists complete with stooges to suck people in, lookouts on every corner and people watching for likely pick pocket targets. I d been on that bridge many times and never seen anything like it. The lack of police (who undoubtedly had more important problems in central London at that time of day) made it all too easy.
I stayed in the Park Plaza hotel opposite it for several weeks, so know that area well, and what you speak of. Was always funny if I was moving briskly in business attire I was ignored, but if I was in causal wear and looking around you would get hassled by people selling stuff and more attention from the romanians