Great aeroplane -I've had a share in mine for 17 years and 1000 hours!
Depends where you are in London but there are instructors with AA5 experience at many airfields around the city; look for long-serving and ex Cabair people. They used them for many years. If you are near Elstree there are specialists there. PM me if you need more.
And get it looked at before purchase by someone who KNOWS the type, not just an engineer who has seen one a couple of times. There's a man who knows the history of virtually every AA5 in the country who can help steer you. Again - PM me
rgsaero