It depends on the cinema. The big chains are worse about it, but they do state on their websites (in small print) how much advert and preview time there is so you could choose to come in a bit later. Our local wee cinema, on the other hand, often shows no adverts or previews at all on its main screen, at least in the first week of a new film being out, and it costs £6.50. You might want to check out smaller/independent cinemas instead of the Odeons and Cineworlds.
ETA: Also, if it was the 3D that costs more. Usually Odeon and Cineworld the tickets are around £8 for an adult evening ticket. The last time I went to the cinema in the US it cost $8 for the matinee and $12.50 for the evening, so that isn't so different.