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I'm sure there used to be a link to a split ticketing site that gave ad revenue to RailUK forums... but I can't find it now. Has it stopped or am I just bad at finding things?
We used to have an affiliate link to Trainsplit which gave us a little bit of revenue when you bought tickets using it but we discontinued it some time ago. Otherwise as @Intermodal says as much as we have anything as being "RailUK Approved" Trainsplit would likely be it for your split ticketing needs.