If a bookmaker has done anything you, a punter, consider wrong, the best first step is to contact the business directly. That’s possible through email or live chat, where you can reach customer support. If that doesn’t resolve the issue, there’s a more drastic step.
While it’s rare that punters will ever have to go beyond contacting the bookmaker directly, it is possible to submit a complaint to the gambling licensing body that issued a gambling license to the betting site. A less drastic step would be to contact the Independent Betting Adjudication Service (IBAS), which resolves disputes independently.