I don't think one can produce a meaningful spreadsheet for a decision between owning and renting.
The cost of ownership is unpredictable once the aircraft is out of warranty, especially as it gets as old as the average UK GA plane is (25 years currently). Yet, unless one flies say 500 hrs/year in which case the fuel and airtime-based maintenance will dominate, unexpected maintenance costs are a huge feature in any calculation.
One could buy a new plane and get a 2-year warranty, and even purchase an extended warranty for some bits. But one pays a price for this in the amount of capital tied up. This capital could be invested. But if you invest all capital, you will die very rich but you will not have had any fun. So that doesn't make any sense either. If one always looked at the alternative returns on capital, one would never do anything. So, buying a new plane has got to be the cheapest way to fly... you just have to find the money
If one disregards the above stuff, it usually works out that the breakpoint between owning and renting is somewhere in the 100-300hr area. But this is almost meaningless unless the client is entirely happy to limit himself to the sort of very limited flying that's possible on self fly hire.
All this is before one gets into really subjective arguments like the value of easy access, ability to take it away for some days or weeks, have it maintained to one's own standards and not "standards" which others are prepared to pay for, knowledge that somebody hasn't bent it and kept quiet about it, the very low marginal hourly cost of flying...
I know almost nobody who keeps reasonably current on self fly hire VFR and I know even fewer people who keep current on self fly hire IFR
Incidentally I am pretty sure that if a flying school/club set up with brand new £150k-£200k planes, they would not have to charge a penny more than the school next door operating 25 year old planes. And this is Avgas for Avgas. Diesel would be even better, though I doubt today's diesels are ready for prime time yet in a training environment.