This is whats called a dodgy casino.. probably unregulated. (not that the UKGC are good for anything). Tons of things need sorting in this industry, dodgy casinos, usually newish ones, will do all sorts of things to avoid paying out.. especially when bonuses are involved.
It would be better if the UKGC just said, the only bonuses you can receive, are free spins, with no wagering requirements.