Ah, OK, healthcare vouchers are already being piloted. They're called Personal Health Budgets.
I think at the moment they're for services like physiotherapy, but obviously once the system is up and running all services can be brought into the system.
Makes sense in a restructured/privatised healthcare system, where "NHS" is simply a brand name under which companies can pay to use.
The rhetoric to shoehorn them in will be the usual, "Phase 1: Don't worry, it's only minor services, you have nothing to complain about. Phase 2: we're expanding it to all services, but the principal is established so you have nothing to complain about."
What I can't find is how the amount of the voucher PHB is allocated. But once the PHB structure is in place it will become trivial to compare the amount each patient is getting, and then we'll see the "she's getting more than me" brigade we know so well ("Wah, your DC has riding therapy/one-to-one TA/priority parking. Me want!").
The word "fair" will doubtless be used to describe, er, giving everyone the same amount of healthcare... But that's OK, because people who are Genuinely Sick? can apply to, oh someone, and see if they can be allocated more vouchers.
Sorry, that's a rather loose summary.
"Health 'voucher' for NHS patients" Telegraph, 2008
Details, trials, bullshit and papers Personal Health Budgets, Dept of Health.