sportingbet is pretty safe now that the earn out delas have been resolved. They are a small listed UK stock and their share price has roughly trebled recently. They were bear raided when their financial position was weaker and finished up oversold.
I have found all of the spbet books - Sporting Odds, sportsbook and sportingbet itself - to be very fair books and quick payers. They are generally fine up to about $1,000 on most sports, above that it depends on the individual line.