I think you need to define 'decent road access'. A lot of VIP's think that because they drive straight out of an airport onto a multilane highway that the access is good. Unfortunately the motorways in the south east of England are well documented rubbish. The M1 south of Luton is a classic multi lane traffic jam. Knowledge of the minor roads gives a distinct advantage time wise although some customers feel a bit exposed.
Go to the Eurocontrol CFMU movement statistics if you have authorised access and you will find that Biggin Hill is consistently the third buisiest corporate airport in the London area. We overtake Stansted and Northolt and sit behind Luton and Farnborough.
Heathrow. Gatwick and Stansted will always be the preferred destinations because of their larger than life image resulting from their airline operation. In reality they want corporate traffic only whilst they have spare capacity. As soon as all runway slots are taken by the airlines the corporates are pushed out. You are the poor relation at these destinations.
The whole purpose of a corporate aircraft is that you can go to the airports the big boys do not and use their flexibility. Sending a corporate flight to Heathrow is ignoring this flexibility. The pax may as well pay top dollar for top class on an airline and scrap the company aircraft instead.