The vast majority of these foreign fighters will be far more of a hinderence than a help to the Ukrainians. They have more than enough people of their own willing to pick up a gun. The vast majority of them will have completed their national service and it will be relatively easy to organise them into units, The problem will be equiping and substaining them.
The last thing they need is a load of foreigners, most of whom are unlikely to speak Ukrainian or Russian, all with different standard of training (if at all). Probably years out of date and totally unfamilar with the Ukrainian military and its way of operating.
The International Brigade in the Spanish Civil War has been modelled as a template, but that lasted from 1936 -1939, and the different contingents had to be organised in national contingents. A lot of them were WW1 veterans and could carry those with no military training (like Jack Jones) as they had some time to train and organise before they were committed to action. Even then their performance was indifferent, with the British, American and German battalions doing quite well, but others not so much.
Look at the performance of the foreign mercenaries who pitched up in Bosnia between 1992 and 1995. The exception could be personnel who have recently left the Armed Forces who have specialist skills that the Ukrainians need. Instructors in weapons systems that we have supplied to them such as Javellin and NLAW, who are able to not only train their guys up, but advise on the best tactical use of such systems.
Another thing is that if these guys get captured and they aren't shot immediately, they will end up somewhere in a Siberian gulag and the British Government will be getting flak from the relatives about the fact that 'they are not doing anything to obtain the release of their little Johnny who only went because Liz Truss gave him the green light.'
The whole thing is a bit of propaganda by the Ukrainian government to show that the whole world is on its side, which is fair enough, but is not really a pratical proposition