It is based on the best scientific measures I could find. I note that the probability of encountering an HIV infected prostitute in the UK (5%)
Telling the forum that 5% of UK Toms have HIV is alarmist. This figure is from straw poll data not gathered scientifically in 2006. You obviously didn't look that hard for robust data!
More up to date and robust data is to be found on P30 of this report from Public Health England
published in 2017 External Link/Members OnlyQuoting the above report "In 2016, 4,491 sex workers eligible for HIV testing attended specialist SHS, 84% of whom were tested for HIV. This testing identified 11 HIV diagnoses (0.3% positivity)"
That's 3,772 Toms HIV tested, 11 of whom were HIV+
Whilst I choose not to BB for all sorts of reasons and I am no defender of the practice, the chances of catching HIV from a random Tom in the UK aren't 1:50,000 as the OP claimed, they are more like 1:850,000
Furthermore the data above is based on a sample of nearly 4,000 people which is a robust enough sample size from which to draw conclusions about a large population, the OP's report quotes a sample size of 120 which just isn't robust.
1 alarmist post, no punts and no intention to punt. Troll?