I don't know the percentage but, yes, I do know what you mean - and so does the CAA.
Leasing an aircraft is perfectly legal, but the CAA is well aware of the 'fiddles' which go on. As with many things, in aviation and elsewhere, 'fiddles' are all very well until there's an investigation and the arrangement is closely examined. An investigation may be triggered by an incident or by a report to the CAA - the latter often by a commercial operator.
If there's an investigation, the CAA will not only check the paperwork thoroughly but also speak to the person named as the lessee. (Or, more accurately, try to - the lessee is under no obligation to answer the CAA's questions.)
If, after investigating, the CAA suspects it was a charter dressed up as a lease and there's evidence to support that suspicion, the CAA is likely to prosecute.
The CAA isn't as active in this area as it could be. I've never understood why, given that they investigate and prosecute very trivial matters.
Tudor Owen
PS
Re your first post -
I haven't heard about the court case you mention, but I know of a report which was investigated earlier this summer.
Everything was in order and nothing came of it.