There are numerous casinos in London, although you'd be hard pressed to find one advertised, as it's not legal for them to advertise nor for them to be listed in directories. Still, look around and they're all over. Keep in mind, though, that in order to be legal, the casinos have to be "members only." That means either you have to pay a fee - albeit a small one - to become a temporary member, or be a guest of a full-time member to get in.
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