Casino’s should show significant terms and conditions up front – IE, make it upfront and clear the important elements of the promotion. If you feel like they’ve not done this and “buried” in the terms section, you should complain to the ASA who will be able to consider your complaint.
This happened to me once on a Nektan Casino, all be it to make it worse, they put the money in my account as cash and then said I wasn’t allowed to withdraw more than the win amount (I was out over 9K as a result). Unfortunately for them, I know what Casino games have the highest progressive contribution rates / License fees attached to them and was able to empty enough of it into the right game to at least feel like I was hurting them in return!
Good luck.