I truly wonder why does EasyJet need to know my ID details in advance for my flight from Prague to London:
It is a requirement of UK customs? If so, why haven't I been asked this information for my EuroStar ticket from Paris to London?
The UK is indeed among the countries requiring “Advance Passenger Information” according to Wikipedia, although I don't recall ever needing to enter my passport details in advance (last time I went there must be a couple of years ago).
Trains are treated differently, probably because you have to clear immigration in France or Belgium, before boarding the train. Not so with air travel: You will be on board long before any contact with the British authorities and are already on UK territory when approaching UK border control. Furthermore, planes come to the UK from all sorts of far away places, not only a couple of friendly and (mostly) reliable countries.