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If someone is travelling TO the UK then yes they should check with a BRITISH Embassy whether they need a visa. That's what they are there for, in part. Their own Embassy will have second hand or zero information. If they needed a visa they would not get it at their own Embassy, but at a British one .
If they are travelling TO Ireland then it's the Irish Embassy they should check with. (The info I gave is from Irish Ferries and is probably unconsciously assuming travel from Ireland to the UK)