There IS some kind of precedent. Betfair have been quietly caving in to UK players over the "happy hour" promo rather than risk have a case end up in court to set that legal precedent. It's not something they want widely known, but it was reported here last year.
In fact, many businesses reconsider when they think a customer is serious about taking legal action, and only the real rogues let it get to court because they intend to ignore the court anyway by racking up loads of CCJs and then liquidating the company to get out of paying. I doubt this casino intends to ignore valid judgements and liquidate the company to get out of paying.
It's true that currently it's a difficult path due to the licensing regime we have at present, but this is a dangerous policy to have in place and actually USE on what appears to be a regular basis given the desire to expand in the UK market. These consumer laws are derived from an EU directive, so all EU member states have to introduce laws that cover the requirements in the directive. The UK has a reputation for being rather "thorough" when it comes to implementing an EU directive, and when there are complaints, the EU is to blame, not the UK government.
One basic thing that many forget is that under UK law, a term deemed "unfair" can be struck down even if it has been read and agreed to by the customer, so this argument does not protect the casino in this case. It is also illegal for a private company to arbitrarily issue "fines", which is what got the banks into trouble over "penalty charges".
To be legal, ALL players would have to be subject to the 5x winnings cap, not just a few that the management can pick at their own discretion. What they can do is to refuse registration to any player they feel deserves being picked for the 5x cap, this is covered by the "right to refuse service", but even this can be a minefield as the clever customer could sue on the grounds of discrimination on grounds of something like race, religion, etc, and faced with such a charge, the business has to prove the discrimination was not for one of these reasons. This often happens when people don't get that plum job they were after, but discover that a white male Caucasian Christian got it instead, despite seeming to have nothing better in the way of qualifications.
Read 
