Is it possible to remain completely airside in London Luton airport when changing between two Easyjet flights? I will only have hand baggage and will already have my boarding pass printed out before going to the airport.
No it's not
All international arrivals are escorted through an isolated corridor to immigration, with no transit corridor available.
Thus, all transit passengers at Luton must comply with the UK's entry (or at least landside transit) requirements.
The UK airports that do offer airside transit are Heathrow, Gatwick and Manchester.