there's already a similar problem for expats with medicare. When you reach 65, Medicare A (hospitalization) is free, although you can't use it outside the US. Medicare B (outpatient), however, you have to pay for, even though you can't use it overseas. Every year you're eligible and don't enroll into pay into Medicare B, there's a 10% addition to your rate in the event you eventually do enroll. The complications for Medigap are the same but even more complex re pre-existing conditions, etc. This applies even though you have the NHS, and even if you also have UK private health insurance, including travel insurance. If you're an expat over 65 who will eventually return to the US you'll probably end up signing up for Medicare B to avoid the penalties, keep your foot in the door, etc. Not pure taxation but same effect - a cost but no need and no benefit.
(The SSA at the embassy tells you to be sure to enroll in medicare A when you reach 65 but it's debatable whether that's very good advice).