US price doesn't include sales tax (?).
Correct, US price is before sales tax. That's the difference. So you were quoting a number *with VAT*. Let's recompute! 20% VAT, 1.6 dollars to the pound.
The UK 85kWh standard model should be a little over (69900+7500) * 1.2 / 1.6 = 58050 pounds sterling, after VAT. 65000 would be a 10% premium, which is, I suppose, plausible for shipping and homologation and cushion for exchange rate fluctuation.
But you said you'd sell the Ampera for a car with 150 miles "real" range and 40K pounds price. The model S base model should actually do that (your speed limits aren't that high), so (49900 + 7500) * 1.2 / 1.6 = 43050 pounds sterling.
So again, the Model S really is very close to the car you're looking for, and Bluestar will probably be the car you're looking for. I can't see the UK Model S coming in at 65K pounds unless Tesla is price-gouging. Which they do; the Signature is frankly overpriced.
I certainly don't blame you for getting a more affordable car. For reference, my Signature is coming in at $103K before delivery fees, and that's without the panoramic roof or rear-facing seats. That's ferociously expensive, and I am *not* getting exactly the car I'd want (I'd buy a compact with differently designed seats), which makes me conflicted about it.