A 300% deposit bonus in the UK is an enticing offer from online casinos and betting sites, allowing players to triple their initial deposit. For instance, if a player deposits £100, they receive an additional £300 in bonus funds, giving them a total of £400 to play with. This type of promotion is designed to attract new customers and enhance their gaming experience. However, players should always check the terms and conditions, including wagering requirements, to fully understand how to utilize the bonus effectively and maximize their chances of winning.