I think there is a list of aerobtaic instructors on the British Aerobatic Association web site (WWW.aerobtaics.org.uk) sorry don't know how to do the hyperlinkythingy.
Who ever you go to, speak to other people first as there are aerobatic instructors and there are aerobatic instructors. My local offer Advanced Aerobatic instruction in their ads but imho the instructor would struggle at beginners!
Alan Cassidy is top man tho and member of the UK Unlimited AErobatic team. Get someone who can do it not just read a book about it.