don't forget to go an have your visa checked (read passport) at a BA check in desk before lining up for security.

In T4 you can OLCI or SSCI and walk through security to have your passport checked at the gate.

In T5 regardless, you need to see a person before lining up, otherwise you'll get to the front of the security line only to be sent off to see someone at BA and then go to the back of the line again (or the frontt if they remember you) which is all hugely pointless but there we go!