I am very sure that there are British special forces in Ukraine.
After one of his visits to Ukraine, Michael Kofman was being interviewed about the possibility of soldiers going in and partway through started laughing and said “if anyone has people there, I’m sure the British do”. The interviewer asked him why he thought that and he laughed even harder and said “let’s just call it one analyst’s guess”.
My guess is that they’re present and that it’s an open secret that they’re there, but that the British government chooses not to officially announce the fact.
Later, they had that German military discussion that leaked and IIRC they said that the British had about 50 people in Ukraine, more than anyone else…IIRC we in the US had 17. The Brits were doing things like helping target British weapons (and French weapons, as IIRC the French didn’t have anyone present). The Germans were talking about having the British target their weapons too if they decided to send Taurus missiles, so that they could avoid putting anyone in-country.