I think I have some answers to this, but since I am in the U.S. and you are in the UK, I am not 100% sure they will be applicable. I think they were using the same chargers but configured differently in the North American cars versus the European ones, since they were made to use 3 phase electricity.
This person's car and mine are from about March 2014 and already has the Gen2 charger in it. And someone else there had a June 2015 car that also had that Gen2. So by early 2014, they had already moved from Gen1 to Gen2. There is a marking difference on them. The Gen1 was made for just 40A. The Gen2 says on it 48A, but it was still being used at 40A in the cars, but here is where I think the UK/Europe part comes in. Does this look like dual chargers to you?
From that comment in the same thread, that person from Norway said that the Gen2 is what they have in their European cars to handle the 11kW power, set up for 3 phase.
So here's my theory: when Tesla first started building and selling the Model S in 2012, they obviously didn't know if they would make it long enough to expand to Europe, and I think they were just using the Gen1, which was probably a simple single phase one. By the time they were going to start building and selling to Europe, I think that's when they shifted to that Gen2, which could be configured either as the 40A single phase for North America, or for the 3 phase in Europe. So I think all of the cars that went to Europe were Gen2 chargers until the April 2016 facelift redesign, which I think is the Gen3 charger.