@Anonymous User
Is there money in the account, is the card a regular bank or credit card not a prepayment card or an Amex card, is the bank a recognised UK bank operating in sterling?
Any or all of these things not being so will cause NowTV to reject the card.
And what is a ‘bf card’?
Set a Payment PIN on your account so that no-one but you can buy memberships on it. Check your bank accounts monthly for any other unexpected payments to Now. That way you can at least nip them in the bud, while you and Now figure out whose fault they are.