While I have no idea where and when board members will be available sept 13 starting 3/4pm in London would probably be best bet for reasonable attendance.
The other limitation is that 21 days notice should be given. However that can be changed if 90% of members agree.
I thought it was 100% of members, but either way that's not difficult since the members are pretty much the board and no-one else, since they haven't been accepting applications.
We had a discussion on IRC about when and where would be a good time to hold it and Saturday afternoon in London was, indeed, the best bet. The 20th may be easier than the 13th (makes no difference to me), but I think that's the last chance - the AGM has to be held before the 23rd (it has to be within 15 months of the last one, which was 23rd June last year). I'm not sure what really happens if the board doesn't call one within that time - they get put in the stocks and have rotten tomatoes thrown at them, probably!