Originally, before deliveries started in the UK, EU Model S accepted 16A (single or 3-phase, so 3.5 or 11kW) unless you paid extra for the second charger which doubled those ratings.
Given the prevalence of 32A single phase chargepoints in the UK, all UK Model S were delivered with the second charger physically fitted, but with a software lock to limit current to 16A on three-phase unless you had paid the extra. So still 11kW base or 22kW if you paid for 2nd charger. This charger design was purely current limited, so you could get up to 24kW out of it if the mains voltage happens to be high.
There were rumours of a hardware fix to allow 32A single phase on single-charger cars, but I'm not convinced it ever appeared.
The new charger appeared on Model X. This supports up to 24A three-phase or 32A single phase (I'm not sure if the single phase behaviour has ever been fully explored). Unlike the old design, there's an overall power limit so you won't see more than 16.5kW out of it. At launch, there was still a paid-for option to get the full 16.5kW rating, so the base model was 11kW. This made the paid option even worse value for money, so nobody well-informed in the UK will have ordered it (I'm not sure how many less well-informed owners there might be).
At around the time of the Model S facelift, the new charger design from Model X appeared in Model S. Again, full 16.5kW was extra cost.
Subsequently, 16.5kW became standard and the extra-cost option disappeared. I don't have an exact date for this.
So:
a) Cars delivered in the last year or so are all 16.5kW max on 3-phase, 7.3kW on single phase.
b) If buying a used facelift car from before the cutoff date, it's very likely to be 11kW software limited.
c) If buying a pre-facelift car from around 2015-16, it's very likely to be "software single charger" and so 11kW (12kW on a good day). Very few will have paid the extra to enable the second charger, though you should still be able to activate it retrospectively at a price.
d) If buying a pre-facelift UK car from 2014/early2015 (ie. before the CHAdeMO adapter), it's quite likely to have dual chargers enabled since there were few Superchargers and the ability to charge at 22kW on Ecotricity sites made the option seem worth having. Still, not everybody took it so some are software limited to 16A (11kW).
e) If contemplating a LHD import from Europe, beware that many were physically single charger and will charge at painfully slow 3.5kW on a typical home or public chargepoint (11kW on three-phase).
