The short answer is that Paris is and
AFAIK always has been about as "accessible" to GA as London is "accessible" to GA
To be fair I think London is much better, if you are really organised you can be at Victoria in about 45 mins from Biggin Hill; 15 minutes taxi to Bromley South station and then if you get on a fast service 16-20 mins into Victoria...I add 10 mins for the likely cumulative time in between different transport modes...
Biggin has customs, AVGAS, is reasonably priced and the staff are very helpful...I just wish it had an equivalent in Paris.
The bizjets can get closer, and they can go into Toussus and get Customs (at some price) because I've seen some on the Biggin departures board. But to a jet client a £500-1000 fee is a non-event.
How long ago was that? The NOTAM of no customs is quite recent. Not that I'd be prepared to pay £1000 for the privilege but how does one find out about these special arrangements? Also for a biz jet needing customs wouldn't they just go to Le Bourget?