It is, yeah.
In case you haven't noticed (I assume you're consuming propaganda 24/7 and thus haven't) UK has acknowledged in early March already that some soldiers have indeed deserted and are probably in Ukraine, and that they will be prosecuted for desertion if/when they return.
https://www.dailymail.co.uk/news/article-10595081/British-soldiers-went-AWOL-fight-Ukraine-prosecuted-DESERTION.html
The fact that one of them has apparently been captured means nothing, as unlike Orks, British soldiers are mentally capable of disobeying orders and being in Ukraine on their own accord, against the laws of their home country.