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.
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.