JetRob
The decision to get B767's is a bit more in depth than "see what they can get".
The original B767 was a -300ER version but the 'variant specification' meant it was quite a rare machine as only 8 were ever in service. I believe for commercial reasons the Board wanted to get "a pair of sisterships" for maintenance savings.
As for which version Jet2 would end up with I think it would be the -300ER because the -200 in Hi-Density config. would struggle to get across the Atlantic if Jet 2 wanted to cram in as many pax above & as much freight below. I don't think they'd go for the -400 as it isn't a type on the UK register. If Jet2 were to be the 1st UK operator of the -400, it would incur HUGE costs.