Putting British troops on the ground in Ukraine once the Ukrainian state truly begins to disintegrate and there’s a need for a backstop force to be deployed. I reckon the capacity of both sides to avoid direct strikes on one another’s territory would hold up, as it has so far even with western missiles and tanks being deployed inside Kursk. I’m thinking UK troops present in a “strictly defensive” or “advisory” capacity such as being stationed along the Belarusian border or along inactive sections of the front, while the Russians continue to avoid strikes outside of Ukraine because as you say there’s no interest there in actually hitting the rest of Europe. There’s no reason that NATO would involve itself if Ukraine wasn’t a NATO state (no legal basis for it), and US unwillingness to back such a move would give enough diplomatic leeway for both sides to say that no fundamental red lines had been crossed.

Of course it would still only be a prelude to a greater escalation, but that seems inevitable anyway if and when Ukraine collapses, and I think there could be a few years of European states putting troops on the ground inside Ukraine to some degree to forestall the next phase of conflict. If Ukraine does just collapse then you can be sure that there will be conscription and a much larger deployment to NATO’s eastern border anyway.