Not sure if it is exactly what you are looking for, but I have been pleased with my Access Toyota lease from Toyota Finance. Might be worth a look, assuming you might be interested in a Toyota of course. I've found the main dealers generally very helpful too.
The car I wanted was £15k my trade in was worth £3k which left £12k but I only had to finance £6k over 3 years. At the end of 3 years I can either a) Return the car and have another one . b) Pay the outstanding balance and keep the car. or c) Simply give it back and walk away.
The car has a guaranteed future value, in 3 years time it will be worth minimum £6k regardless of actual market value (which could be more or less) that value can be used as trade in for a new car
I don't have to wait 3 years though, I'm only 11 months in but I am still free to trade it for another one, buy it, or in another 6 months I can give it back.
There are downsides, as already pointed out, Any scuffs or marks you have to get sorted out before you hand it back, if a stone goes through the A/C rad an causes £700's worth of damage then you have to get it fixed, wheras if it was you own car you might not bother (speaking from personal experience)
You are limited on mileage, although I had no problem arranging to do 20,000 a year on a diesel avensis, they were a bit reluctant when I tried to get a quote to do the same mileage on a GT86 The more miles you do the more it costs you.