Prepaid cards aren’t protected by the Financial Services Compensation Scheme (FSCS) in the same way bank and savings accounts are, for example.
But if your card provider is ring-fenced by a licensed bank, your money should be protected if they go bust. Ring fencing means that the card provider is required to ‘safeguard’ your cash by holding it in a separate bank account from their own funds.
Just be aware that if the ring-fencing bank went out of business, you might not get the money on your card back.