It's a Heathrow specific process. You're mug entered the Heathrow security system and must be expunged. This normally happens as you board your plane. Not an issue if you've entered T5 from another UK airport, since you never entered the Heathrow security system.

I was seriously affected by this years ago when Heathrow practically closed for 3 hours on a Friday afternoon for a security test run . We were all " boarded" i.e. expunged from T5 security system, and then "guided" back out into T5 departure checkin. Hundreds of us.