Depositing at a casino prior to making a withdrawal is a legal requirement these days in the United Kingdom. It helps iGaming operators stay on the right side of anti-money laundering regulations.
No deposit bonuses simply mean that you don’t need to deposit to claim the bonus. If you are lucky enough to beat the wagering requirements, you will need still need to make a small deposit to register your bank details at the casino and to prove that your account name matches the name that you initially registered with.