To be able to legaly sell your goods your 'friends' would need to have made reasonable attempts to contact you and get you to collect them. Only then (if you couldn't be contacted or if you failed to collect the goods) could they sell them. Even if they did, they'd be obliged to seek the best price for them and then, upon your reappearance, to give you the money that they'd got for them.
[Torts (Interference with Goods) Act 1977]
The law states that: "A person is guilty of theft if he dishonestly appropriates property belonging to another with the intention of permanently depriving the other of it"
. . . and that: "Any assumption by a person of the rights of an owner amounts to an appropriation, and this includes, where he has come by the property (innocently or not) without stealing it".
[Theft Act 1968]
So, at first sight at least, it seems that your 'friends' are guilty of theft. However they would have a valid defence if they could show that they genuinely believed that you wouldn't mind them appropriating your property. (e.g. because they genuinely believed that you'd abandoned it and had no intention of returning to collect it). Because of that the police/CPS might be reluctant to get involved.However it might still be worth having a word with your local Community Constable (if you've got one).
The threat of civil action might be worth a try though. You'd need to write a (dated) letter, showing the full name and address of whoever it was that you were threatening to take to court, as well as your own name and address. The letter should clearly state the amount that you're demanding and the reason that your're making the demand (i.e. because of what you've had to fork out to replace the goods). It should also clearly state that if you don't receive payment within a specified period (I suggest 21 days) you will commence legal proceedings to get your money. You should send your letter by Royal Mail's 'Signed For' service and ensure that you keep a copy of it.
The simple threat of legal proceedings might be enough for you to get compensation for your loss. If you don't get it you'll have to decide whether or not you actually want to initiate court action. If you decide to do so, you can do it online here:
https://www.moneyclaim.gov.uk/web/mcol/welcome