£2 billion spent on foreign patients treated on the NHS.
Not as much as that say Labour.
May be more than that says the Conservatives.
I would accept with a marked degree of skepticism ANY statistic or report concerning the NHS from ANY politician and unfortunately any medical source.
If you, as you say, are a compassionate, philanthropic nation that distances itself from the uncaring countries of Europe and especially the US as healthcare is concerned, then stop moaning and pay up.
Of course one could make it clear that health insurance should be a pre-requisite for entry into the UK and without it one would need to pay for medical care, but that would cause to many problems, administrative and emotional, although other countries seem to manage.