T5 to T5 is an easy connection, well do able in under 60 minutes from Y never mind ninety from F
are your friends connecting with you? are you guesting them in a lounge or going to "dump" them after security?
If you are spending time together in the terminal or they are exiting at London then I would wait. If you are just going to meet up say hi and then leave them 10 minutes later when you are through security I would not bother