you are right suzie - it is horrendously complex in real life.
GPs feel that a) it is not their job to check the status of patients and b) they do not have the time, the staff, etc, to do the checks- extra staff would have to be appointed and trained - and who would pay for those? + extra accomodation. a dedicated room, etc, etc.
Secondly, if you do have sick people in the community- be it HIV, TB, measles, or any communicable disease- is it best to treat and therefore control the spread, educate, etc- or not treat and allow risk of contamination.
It sounds simple, but it is certainly not.
However, it is ridiculous that overseas people with holiday insurance who offer to pay, as they are fully covered- are told 'not to worry, we can't be bothered' ... which I have witnessed with my own eyes at A&E in Bedford.