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People going to fight in the Spanish Civil War were warned of the risk of prosecution, but no prosecutions were actually brought. Presumably on the basis that the prospect of getting a jury to convict someone of fighting fascists was extremely remote and therefore it wasn't worth the effort. I suspect the same would apply here, but it's still technically illegal for a UK citizen to go to fight in Ukraine.
Looks like he deleted it
Can see the discussion that started from it:
https://twitter.com/BarristerSecret/status/1498055777373798413
Was citing a law that says UK citizens can't fight in a war in which the UK is not at war with one of the parties, they seem to be arguing it isn't relevant as Russia and Ukraine haven't declared war...
David Davis was on LBC citing the same Law earlier