Looks like the CAA can't be bothered to impose any additional limitations on operators using NHT:
The Battle of London - London Biggin Hill Airport versus London Northolt Airport - CJI Main Site
There's clearly a whole bunch of non-compliances and infringements there and yet it's deemed to be O.K. to use the airfield with the current MAA (military) numbers without any adjustment or factoring for equivalent civil compliance were it to be a licensed airport. And yet it's fundamentally a civil airport - 95% civilian use.
Business aviation operators don't want to lose access were the runway usable or declared lengths reduced to what they should be for civil use, whilst the CAA have no ability to tell the MOD what to do at the airport and don't deem it unsafe enough to impose any constraints on civil use.
I would suggest that if a considered opinion of a specialist airport safety outfit such as Mott MacDonald says as a minimum, some 300m plus should be taken off the LDA (landing distance) for 'safe' use, then somebody somewhere is turning a blind eye to the whole matter - all too difficult with no precedent in the UK in terms of how to deal with the issue. That makes one hell of a difference to a larger, heavier business jet on a wet runway with known poor friction characteristics.