A welcome bonus in the UK is usually a match deposit bonus, where your deposit is matched with a percentage bonus. Most casinos offer at least a 100% match bonus on your first deposit, with a maximum bonus amount allocated.
The bonus amount is subject to wagering requirements, which means that you have to turn over the bonus funds a certain number of times before you can withdraw any winnings. This turnover is usually between 20 and 40 times the bonus amount, depending on the casino.
In some cases, such as free spins promotions, the bonus amount is added to your account as bonus funds, while free spin promotions allow you to play specified games for a set number of spins.