Hi ockerdoc,
The following is quoted from the William Hill guide.
For Beginners: Automatic Bankroll Protection
In collaboration with PokerStrategy.com, William Hill offers an automatic bankroll protection for beginners. When starting out with your $50 starting capital, you will only be able to play in the following limits:
* Fixed Limit Hold'em: Up to $0.10/$0.20
* No Limit Hold'em: Up to $0.05/$0.10
* Sit and Goes Texas Hold'em: Up to $1 buy-in (not including entry-fee)
Sit and Go heads-up tournaments are not supported, since they are not suitable for beginners and we therefore don't offer a beginners' strategy for this game type.
These measures are consistent with our recommended bankroll management method and will help you get through the introductory phase with a profit. The automatic bankroll protection will either be lifted after you've generated 50$ in rake or tournament fees or after 30 days of actively playing for real money at William Hill.
It will also be lifted when you deposit at least $100 on top of your free $50 starting capital.
As you've mentioned that you deposited $300, it appears that this protection should no longer apply. I would recommend contacting William Hill's own support team as there may be something else that I'm not aware of. They should be able to help you.
Good luck & best regards,
Bart