London Metropolitan University (formerly Guildhall), next to the Tower of London. £25ph solo, (£20 for students and ex-students).
It's configured as a Seneca 1 and is kept functional, but is very old. People who are not known to the department are required to do a quick check ride at £75ph before flying the thing solo!!!
Splat and I go fairly regularly and repair to the pub to moan about the state of the job market afterwards