Is a Secured Mastercard available in the UK???

By definition, it's not a secured credit card. For most purposes, you would be better off with a debit card: Delta or Switch. Some car hire firms won't accept them for a deposit. In that case, perhaps you can get a secondary card on the account of a more solvent and creditworthy relative on the promise you will never use it except as a car hire deposit. You can then pay for the car hire with your debit card.

Or you can do the same with a US$ secured credit card (as to which the credit limit is exactly the same as your securing deposit).