Depending on the aircraft's owners wishes / agreement, you can fly their aircraft and pay the "costs" - under the terms of AIC60(W128)/2006 - without buying a share.
I'm not sure that this is strictly true. Yes there is a way to get exempted from the "No hire & reward" rule for a Permit aircraft in order to train on it, but I think you'll find that you have to have a very good reason for the CAA to grant an exemption i.e either having bought an aircraft and then require training on it by a third party or about to buy a share in said aircraft. In the last case, I am reliably informed that the CAA grants a 'one flight only' exemption. Guess how I know?
A good example would be if you were fortunate enough to be able to buy a single seat Mk9 Spitfire and had no previous Spitfire time, you could then buy time in a 2 seat Spitfire trainer to get current on type etc etc.