I find it funny, how concerts in London tend to run like clockwork. So much so that they rarely stray from this schedule:
|19:00||Second support starts|
|19:30||Second support ends|
|19:45||Main support starts|
|20:30||Main support ends|
Doors normally open at 7, so it's not unusual that you have missed most of the first act by the time you enter the venue. Fortunately, some venues do open earlier (6pm) if there's going to be a second support.
In any case: super handy to plan around.