Weather in September?

[preznto]

Hey all, I've made the mistake of agreeing to organize my sister's wedding that is scheduled for September. My sister is a classic bridezilla and I've already regretted taking on this obligation, but there's no going back at this point. 

She wants to get married in September in London. But, she wants to have an outside wedding where she plans on doing the whole event under open skies. In September. In London....I've tried talking some sense into her, but to no avail. She's set in her ways and we're left scrambling to make it happen. I am yet to book the outdoor venue (I'm cutting it close, but it's a September wedding in London, there's no waiting list for that lol) as I'm not sure what the weather will be like in the upcoming month.

My question is whether it's possible to get a few weeks worth of heads up on any major weather formations? I'm already looking to book a sliding reservation at one of these http://www.headlinecasinos.com/features/londons-five-best-casinos/ (don't ask me why she wants to have a reception at a casino), so I can just tell her to pound sand. However, I'd like to give it a shot if anyone here can tell me whether my idea is too far out. Oh, and we're from the US so I'd really like to nail this if possible. Thanks for any help in advance, I appreciate y'all!