The good news is that .co.uk is one of the more active extensions for gambling stuff. I've done a lot of sales in gambling and a lot of my customers have been in the UK.
The bad news is that if you look at other extensions available, "safe" has tended to work nicely with .org given the trust factor associated with .org, and not only is safebetting.org available, but it's NEVER been taken. To me at least, it signifies a bit of weakness in the term.
I do think if you use it and put up something relevant to the name, it can work out. But on its own, I don't think it would have much value.