when u arrived Lond, there's plenty of "Ticket Box", normally like Phanton will reduced to 27, Les Mis to 35, etc any ticket bos is ok, don't worry, u can find normally near Oxford Street or in Leister Square.
Bu don't expect the atmosphere will lke u experience in Culture Center in HK, u will feel like in Mong Kok Cinema, as it's so common thing to see musical. Also, London always have a set meal "Pre threater meal + musical" sth like thta, u can book thr online, personal expereince,
www.lastminutes.com, u can find a lot, ok?
Lastly, don't compare with HK and hope u enjoy here in London.
I'm living in E London.