Without sounding defensive..... there is indeed very little difference, short haul, between EZY and TUI. Operationally, they are different in places but joe public doesn't recognise those.
Thomas Cook began taking allocation on EZY several years ago now. A few people were a bit suspicious ("i've seen Airline") but most were pleasantly surprised...EZY's onboard soft product is pretty decent. Furthermore, EZY flight times were generally better as they don't do that many true overnight flights and as such, there was anecdotal evidence that the holiday cost was actually a little more expensive than those flying with TCX.