I don't live in the UK, so I'm going to ask -- is this serious? I know in the US there's about 15 minutes of previews/commercials after the presentation starts, but for the 30 min prior to the screening there's the "First Look" or some other less preview/ad thing running. That would make 40 minutes but only if you came significantly before showtime.
I've been to cinemas in a few places around the UK, and it's been The Twenty[0] - 10 mins of ads, then 10 mins of trailers, from the time the show is scheduled to start, before the movie - for well over a decade. However, of late at my local multiplex, it's definitely stretched out to more like 15/15, with 30 mins between the posted start of the show, and the start of the actual movie.
[In London at least, not sure about other places] Ranges from 20 to 40 mins of ads and trailers, although I've only heard of people complaining about the upper extreme and never personally experienced it.
Simple way out is to walk in to the cinema hall about 15 mins after the scheduled start of the movie - works for me every time.
I wouldn't be surprised to learn that they raised the timeframe from 20 to 30/40, and probably factored some randomness in, just to defy that sort of strategy.
