Yes. All airlines prohibit use of electronic devices during the departure and approach phases. That is to say, once the seat belt signs are off (as caber0 says) then you can use it but, once the aircraft starts to descend, be ready for them to ask you to shut the unit down.
Dependent on the your departure point in the UK ("Near EGNL") and destination then the amount of time that you can use the laptop may not be that long, perhaps an hour and plan not to be able to eat or drink whilst using it!