I thought I would add my knowledge of processing here.
Gambling transactions is classed as high risk transaction by the processors. High risk, because more chargebacks are expected. For this reason, some processors will demand that CB rates be anywhere from 1-2% and no more. Going over that amount can force the processor to hold the casino’s funds or a % of it, for any period of time, if they feel they need to cover those CBs.
I believe there is also policy in place by Visa or MC, or both, where a person can dispute up to 50 GBP, and have it automatically processed as a CB. In the past, a casino could dispute that requested CB by providing documentation that it was a legitimate deposit.
Policies are always changing, so I am not sure if these still apply.