Sadly its pretty unlikely the police will do anything. If you ever read through the threads of people struggling to get their horses back that have been sold on loan, you will know that the police tend to back off - as it very often comes down to 1 persons word against anothers. There is also no legal documentation to prove who a horse belongs to. If, like me, you blissfully assumed your passport was such a document, you are sadly wrong.
All this wretched woman would need to say is that she owned said horse, or take it lieu of a debt, or had been given it.
It would appear the only way to make some effort towards safeguarding a horse out on loan is to register this with NED, register it as on loan with the passport - and while the legal bods have said it probably would not stand upto legal scrutiny, a firm contract stipuling the horse is on loan and cannot be sold.
I did ask what the legal ramifications were of having a list of names which you could check potential loaners against - say if someone like MHOL was holding it but realistically, I could see that probably wouldn't help address much of the issue and would open a lot of legal issues.
If you look at FB, there is a page - in memory of Millie. Ignore some of the more hysterical postings on it, but the fact remains that this horse died in apalling conditions whilst on loan - and safeguarded and the young girl (15) who did this is STILL taking horses on! and blaming everyone else for the state that Millie got into (the before and after pictures are heartbreaking)