I call BS.
From Civil Aviation Authority website:
Portable Electronic Devices
The use of portable electronic devices for sending and receiving data
and voice calls is not permitted on board aircraft after they have
closed their doors.
It may be possible to use a portable electronic device that has a
transmitting facility in-flight, provided the transmitting facility is
de-activated. This is often referred to as ‘flight safe mode’ or
Some airlines allow customers to use portable electronic devices for
transmitting data and calls if the aircraft is fitted with a system
that supports this, but there may charge for this facility.
If you are flying with a low-cost airline or one that provides additional paid-for "services" on board (meals, drinks, sky shops, entertainment) then this airline "policy" makes economics sense (to them), but you can point out that since the Aviation Authority explicitly allows usage of devices in non-transmitting mode, they might as well STFU and leave you alone.
If you are an EU citizen, I'm sure your local European Consumer Center would like to hear from you. You might also want to check the Is it Fair? initiative, where there are some helpful information on how to signal about unfair business practices.