I use onsite purely for the convenience, having time and time again suffered the inconvenience of offsite parking, particularly those private company offsite ones (which are the ones we can get very, very good discounts on via work). I might not like spending more than necessary, but sometimes its just worth the extra £20-30 for the reduced hassle.
You seem to be very unlucky in much of your dealings with UK companies TB, as the experiences you frequently describe here rarely match mine. 
The last one - the 90 minute wait for the coach to take us back to the car in the early hours was a killer. We'd been on one of those Nile cruises to see the ancient sites, and I forgot one morning to use bottled water to brush my teeth

. After 90mins waiting for the stupid bus, the poo-glue tablets were definitely wearing off.
I think we actually landed at something like 1am, and got home at 6am. Its a 90-100 minute journey at that time. Had I parked onsite, I would have been at home by the time I started the car in the offsite parking.
It was after than one that I started being less tight and paying for onsite parking, and what a revelation it was.
Obviously, at some airports you have no choice. You'd have to be mentally retarded to use an offsite carpark at Luton for a morning flight. That's why onsite parking is so outrageously expensive there - prebooked, £85 a week (8 days), £207 without booking in the Mid term. Even the offsites are £50... ...although even the airlines warn of using offsites there, allow an additional 2hrs

Guess who's using an offsite at Luton next month

(I'm not driving, so just had to go with the flow)