To be honest I've can't recall having ever met any UK transwoman who either admitted to having done sex work or who have said they were HIV positive. I can think of a few I knew through the net who had done/do sex work but they were american (and there was the girl who was featured on that fly-on-the-wall TV program, set around Kings-X), and only one of those was HIV positive - even in America not all srs surgeons will operate if you are HIV positive. I suspect the answere you'll get depends a lot on what your sampling frame is - and for this population there are just no good sampling frames.