Chris the primary target market are the British Gambling whales
Well I can’t guarantee that anywhere you try will bring you any whales unfortunately.
If I were you I would consider targeting the demographic of individuals who frequent online casinos.
What kind of site are you promoting?
What is the audience demographic you are targeting? (Age, gender, hobbies etc)
In the UK the big casinos have a dual strategy of online and offline promotion. The idea being that the individual trusts the brand when they find it online (via brand recognition).
IMO you need deep pockets to make this strategy work.