I do something very similar - in my case a choice of two 4-seater shares at Booker and Cranfield, each of which works out a little over GBP100/hr. I claim back from my employer at standard HMRC rates as if driven. I take a small financial hit and also accept the risk of occasionally having to spend a night in a hotel at my own cost. The payback for me is the flying and the time I get back.
For example last month I had two meetings in a day at Prestwick, flew there and back in the day - ending up down about 150 quid, but gained by about 6 hours flying and an extra night in my own bed. To me, and my boss, that's an okay set of outcomes.
A couple of further thoughts. (1) CAA allow their own staff to use light aircraft for business travel. Is it possible to find out what their staff policy is, since that must surely be acceptable elsewhere. (2) I wonder if there's scope for a cost share /seat share network of people who use light aircraft for business trips. I've certainly done this informally with a friend who works elsewhere but occasionally has business trips to the same place that can be made to co-incide.
G