OK, just been having a chat with my Dad who fought the law with me (and has a better understanding). He agrees that you are performing and according to the letter of the law this would make you licenceable (incidentally even rehearsing music in front of an audience requires a licence since Nov 2005).
However it really is all very confused in the UK at the moment because the new law is very muddled and ill concieved. Also it is still in its infancy so it is currently open to interpretation by the various licencing authorities, so you'll probably get away with it.
I also don't think that this question is confined to matters of the street, technically a restaurant/pub would probably need a licence to have entertainment (eg. a magician), so where does this leave you if you're 'rehearsing' a trick for your mates down the pub?
As you can see it's pretty ridiculous, but then so's the Government's whole idea of 'Regulated Entertainment' (as experts have been trying to tell them for the last couple of years).
The long and the short of it is this imho;
If you're really concerned how this affects you then ask your local authorites, but remember although it's no defence in the eyes of the law ignorance can be bliss and by asking the authorities you're also asking them to make a judgement that they may not even have considered making.
