I don't know what's available in the UK, but I've had good luck with Alumilite. Ultimately, though, any two-part silicone-based resin works more or less the same-- the technique in casting has to do with how you pour the moulds than the actual stuff you're using. You have to go slow with it, and make sure you don't have any air bubbles; otherwise, the whole casting will be ruined.
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