Interesting question.
I know of several helicopters that have been "stolen" in the UK. Mainly due to disputes between owners, part owners, finance companies, banks etc. etc. However, in all the cases I know of the machines were eventually recovered.
I know of one machine that was “lent” to a good friend of the owners for a couple of days. Over a year later it was found in a barn a good distance from it’s original home. God knows how many hours were put on it. Obviously the friendship was reviewed. The borrower is now I believe holidaying at Her Majesty’s pleasure due to an importation disagreement with Customs and Excise.
Another machine I know was regularly flown by a person claiming to be part owner of it. It was always put back in the same place and refuelled but the real, full owner, was completely unaware of what was happening. I know this is not technically theft but definitely not on the right side of the law.
Another machine was refinanced by a potential owner, without the real owners consent. Potential owner disappeared with best part of 300k. The finance company came to repossess the machine. Shall we say the legitimate owner was a little surprised to find somebody hauling his machine away.
Be wary of who you deal with. There are a lot of crooks in this business.
You'd be surprised at how uninterested the authorities (especially the CAA) are in such cases. The Police do peruse as far as they can but I feel its seen as a case of rich people falling out over their toys.