I find it funny, how concerts in London tend to run like clockwork. So much so that they rarely stray from this schedule:

Time
19:00	Second support starts
19:30	Second support ends
19:45	Main support starts
20:30	Main support ends
21:00	Headliner starts
22:30	Headliner 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.