JXC
A lot of good stuff above but a few errors, if the aircraft is maintaned to Public Transport standards then it can be used for instruction.
The old system of public & private C of A is no longer within EASA with all aircraft being maintaned to the same maintenance schedule, however if the 50 hour check is carried out by the owner the aircraft can't be used for instruction untill the next check carried out by a licenced/approved engineer.
If the aircraft has an engine that is working past the TBO+20% it cant be used for instruction.
Insurance is the usual issue that the British use to stop any new idea that reprisents any sort of change, in this case it is unlikely to be an issue as almost all policys issued in the UK cover the owner when taking instruction (just as most cover any licenced engineer for ground running, taxing and flight testing if he has a PPL)
Most approved training company's will drag out all sorts of "rules & regulations" to stop you using your own aircraft but the real reason for this is that the price they charge is an all in one and using your aircraft will reduce the (already thin) proffit margin.
The down sides to ownership are well made in posts above, I did my hour building by buying a share in an aircraft and it worked well but I found the ownwership a little variable....................... As a licenced engineer I found that I seemed to own 100% when the aircraft was U/S but only 20% when the aircraft was servicable!
Please search the website for some of my other posts on this subject as these may be of interest.