According to the 12z GFS, 1000-500 thicknesses peaked at 569dam over London
today.

http://217.160.75.104/pics/MU_London_avn.png
(or http://tinypic.com/5ebrza once the above chart changes later tonight)

Martin Rowley's excellent usw FAQ gives the record as being 568dam for May
over Kent/East Sussex, so it's a close one. The 12z Herstmonceux ascent gave
a thickness of 566.5dam, which matches the GFS forecast for London at that
time...