Originally Posted by
lhrsfo
15 minutes before boarding. LH usually starts boarding 25 minutes prior to departure.
As Stewie mentioned the moving walkways are only for heading to T2B so when your walk back will be unassisted (you might get lucky and catch a buggy ride) and it is a pretty long walk....15 minutes would be at a pretty good pace. Taking the lifts rather than the escalators should save you a couple of minutes.