I doubt it was the "little" bizjets that caused your delay.
From memory it takes at least 24 hours notice to book that slot and lots of penalties for missing it. (time and possible financial penalties for breaking an airport slot).
Bizjets prefer to use any where else than LHR or LGW. Northolt's got it's own set of unique hassles and can be difficult to get slots into there as well.
Farnborough can be a problem too although I haven't operated out of there in ages. (Great FBO though). Biggin Hill's not the biggest and that leaves Luton.
If the pax want to use LHR for a connecting flight, then they do although they are advised of the possible pitfalls.