A debit card takes money straight from your current account, which is exactly why it is the default at British casinos. Since 14 April 2020, licensed operators have not been allowed to accept credit card payments for gambling, so if you try to deposit with a credit card at a licensed site it should be declined.
Withdrawals are normally paid back to the card you deposited with, and the timing depends on the casino's own processing and your bank, so check the cashier page for the operator's stated times before you deposit.
Debit cards are also where bank gambling blocks work best. Most UK banks and building societies now let you block gambling transactions on your card, and some, such as Monzo, make you wait out a cooling-off period before you can switch the block off again. If you are worried about your spending, this is one of the simplest tools available.
Fees for card deposits are set by each casino, so read the payment terms rather than assuming they are free.
Source: www.gamblingcommission.gov.uk