The london meetup does however tend to pull people from outside london. Hold the AGM any other day and you are likely to get lower attendance than on the 14th. If the AGM is to be held on any day other than the 14th we need a date kinda like by Friday to sort out the issues of having two London events we would like people to turn up to in one month.
If it was going to be on the 14th, it really needed to have been announced last Saturday. If it's going to be the 20th (which would be my recommendation, although the 21st would work for me as long as it's not too early), then it needs to be announced by this Saturday. The board really do need to hurry up about this or they'll be in violation of the Article of Association. There is the possibility of getting all the members (I looked it up, it is 100% required) to agree to hold it without 21 clear days notice, but they really shouldn't be relying on that - even if no-one says "no", what if they can't get hold of someone in time? There is no clause in the Articles saying you don't need someone's permission if they don't answer the phone.