The UK regulates "events" far more tightly than France. Most big public events have to go through a local authority "Safety Advisory Group", which often imposes significant professional stewarding and (sometimes) policing requirements, which cost an awful lot of money. If the event is licensable it's inevitable, if it's an oddity like Audax then they don't have their claws into you to the same extent, however any organised event places a duty of care on the organisers, and it is generally felt that abiding by the wishes of a SAG is jolly good idea should liability for a nasty become an issue. The bigger the event, the higher the stakes, so the organisers seem to be, very sensibly, keeping things manageable.
I'd suggest (and it's no more than my personal view) that an event involving the sort of risks that LEL does for more than 1000 participants should be run by a professional event management company.
Staying below the radar, whilst employing a proper and thorough risk assessment, is an understandable policy.