First might be a different ballgame, but even if they already provide you with both physical BP at Glasgow Airport, it might still be necessary to go through the Doc-Check at LHR.

Happened to me last year while flying VIE-IST-TFU. In Vienna they already handed me over the BP for both flights, but thank god I queued again at IST Airport, because they had to register TWOV before the departure of the actual China bound flight with Sichuan Airlines.