I suspect the airbag lifetime depends on temperature extremes where people live, as well as how often they are used.
As far as getting the car moved, did you actually try jacking the body a little bit & inflating the bags ?
Also you said the car is in Salisbury which is not that far from Bugley, even at 35-40mph. I drove mine back from Basingstoke to Langley on the bumps after a compressor airline failure. My spare in the boot didn't have the short line with it . . and yes horrible drive but it's fine if you keep the speed down. Car is still on same airbags.