You had to complete the form online at Antalya airport prior to checking in. If you didn’t complete it you couldn’t check in.
Upon arrival at Gatwick, prior to going through passport control, police were checking that every passenger had completed the form.
Nobody could have got on that flight without completing the online form.
This isn’t the case for every flight into the U.K. though. I recently flew to the U.K. and the airline just asked for passport and boarding card at the gate as usual. I then used the eGates. Exactly the same with a friend on a different flight.