If you have time before going to Heathrow, a side trip to your local surgery might be in order.
Seems to me that the highest probability (higher than tube trains, sports stadia/bars - but NOT airports/aircraft) location for coming into contact with swine flu is in a GP practice - whether its the waiting room, the toilet or just brushing past people on the way in/out to the carpark.
Maybe pax should be asked if they've visited their doc lately. If so they potentially reach the level of contamination risk that they would have in coming into recent contact with cabin crew/ground staff. Surely your average GP is riddled with the virus by now.