I'd start off by working on the horse racing forum.
Then I'd add a horse racing site around it.
Then I'd start adding other sports to make a slow and gradual transition to a website that covers all sports.
It wouldn't be the first sports betting site that covers sports that are interesting to the UK audience - so that's your rug that ties the room together.
Casino is a different story altogether. The site should be built from scratch.
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