Although I have a tenuous grasp of the law relating to financial transactions, every fibre of my being cries out that this is a tenancy deposit, not 'property' and should be protected.
Isn't it the same as giving someone your debit card and PIN number (not recommended, I know), or uncashed travellers' cheques, or a load of euros? They are all means of allowing the recipient to relieve you of pounds sterling, although until exchanged/used to get money out, they are just bits of paper or plastic. It is not the same as a coconut (and I hope someone tells George Osborne that before he reads this forum, gets the wrong end of the stick and starts paying teachers and doctors in coconuts or mangoes).
If it looks, smells and behaves like a deposit...
I have dreamed in my life, dreams that have stayed with me ever after, and changed my ideas; they have gone through and through me, like wine through water, and altered the colour of my mind. Emily Bronte