This is going to be very difficult to police. Most airports have a common security check, regardless of the airline or destination. It would be easy for someone with a non UK airline and destination to bring a laptop through and then give it to a UK bound passenger.
The only way to police this is to have an extra security check at the gate, including a scanner. How long will it take to set this up, and how many gates have the space to do this? What do the airline staff do if they find a laptop as the passenger's bag will already be in the hold?