Victorgolf
You are quite correct in thinking that it would be unlikely to get £ 35,000 for a Cessna 152 in the current market but if you are running a business you have to maintain your aircraft if you want to maintain a good serviceability rate.
I can tell you that a £ 16,000 C152 will have a high time ( if not on condition) engine, old paint, poor interior, McCaully wheels & brakes, old radios and a string of other things about to go wrong. It won't take you long to add up the cost of fixing 60 % of those items and it will add up to IRO £20,000.
It may be that your expectation of flying is to hire a tatty under maintained aircraft that smells like some sort of pond life resides in them carpets, I think that my customers deserve better. That is why the aircraft that I rent are worth £ 35,000 to me, because that is what it would cost to replace one of them.