Problems with buying tickets...

Hi, it's the first time that I post on this message board, I'm from Barcelona so forgive me if my english is not correct!

Next august I'll be in Bristol and I want to go to London in order to see some musicals (specially PHANTOM and GUYS AND DOLLS) but I'm not sure about how to buy the tickets...the main problem is that I won't be staying in London, so I would like to go there, see the musical and return to Bristol, and I wonder if is possible to buy the ticket the same day of the show.

I know that I can buy them online, but I don't know exactly when i'll be able to go so I'd prefer to buy the ticket directly in the theatre..will I find tickets or they will be sold out?

I hope you help me, because it's the first time that I go to the UK, I'm going alone and I don't want to miss it!!!!! Thank you very much!

Thank you for your reply. I've finally purchased the tickets online because I didn't want to miss it...Have you ever been to Her Majesty's? I have a seat on Stalls, Row P, do you think it's ok?. Thank you again!