Come on, Grim, dont worry too much.
You're just going out for a movie. Just make sure you won't be having Kebabs or whatever else that could make you seem full of shit before the show, and you'll be ok...
Just expect her to only want to take advantage of you for a free cinema-ticket, and anything more is bonus
She's english, right!? English girls are reportedly easy slags, so just don't say anything stupid (which in your case means you would be acting like a mute

JK!

) and you're probably already in.
Don't worry man. Just go watch a movie and focus on the movie and enjoying that, instead of constantly being absentminded and thinking about her...