I didn't see it, nor am I very familiar with the British Equity rules, but shouldn't there be a note or an announcement if the understudy is playing the role? There's always been a note between the program when I've seen a show in London and there's been an understudy playing some role. But they may have changed that.
There will always be some kind of notification of any changes in the principal cast of the show at any given performance. In the case of Phantom, there is a board in the lobby with a full character/role list, and the corresponding performer for that performance. This is specifically the case for Phantom in London- other shows do it differently- some make an announcement, some a program insert etc- it differs from venue to venue.
If there was no "Ladies and gentlemen - in tonights performance of The Phantom of the Opera, The Phantom will be played by [someone other than Earl Carpenter]" then it was Earl Carpenter.
Phantom doesnt do pre-show announcements about casts anymore, hasnt done for a few years. If the phantom you saw was slightly overweight, then it was not the standby matthew camelle, it was the understudy for the phantom, rohan tickell (also the principal piangi).