For most online casinos, you can deposit using a debit card without incurring any fees. The card issuer for standard UK debit cards tied to a bank should not charge you any transaction fees for a deposit. The only instance you will need to check this is when you are using a pre-paid debit card. You can get prepaid debit cards from a lot of companies now, but the terms of the card may be that they charge to make transactions. PayPal now offers a debit card service for businesses, and there are charges for each transaction you make.
You should also be able to withdraw for free back to your bank account when you use a debit card. When you are using a debit card, this may differ, as some operators may charge you a fee or your card provider may also add on a fee for this type of transaction. This varies greatly between debit card casinos, but fees can be anything between 1% and 5%. You will need to log on to your chosen debit card casino and go to the terms and conditions page to have a look at the fees.