Use the handbrake.
Never mind what anyone else here says, it's in the Highway Code at Rule 114:
Spot on. Not using it will get you minor faults in your test.
Driving: The Essential Skills (TES, 2010/11 version) which is effectively the syllabus that learners should be taught from says:
You should normally apply the parking brake whenever the vehicle is stationary.
Apply the parking brake and put the gear lever into neutral when youre stopped at traffic lights or queuing behind other vehicles, unless the wait is likely to be very short.
Your foot could easily slip off the footbrake if, for example, your shoes are wet or if youre bumped from behind. You could then be pushed into another vehicle or a pedestrian.
The use of the parking brake is even more important in vehicles fitted with automatic transmission. The parking brake will avoid
the possibility of creep
the vehicle surging forward if the accelerator is pressed accidentally while in D (Drive).
You are going to be marked on the use of the cars controls on your test, and if you dont use the handbrake in a situation where really you should then you will pick up a driver fault at the very least. You might even pick up a serious or dangerous fault if, for example, you roll back towards someone or over a give way line towards traffic.