If we get to a situation where Wales, NI and Scotland do it then I bet BJ will cave in and do it, probably announcing at something like 8pm on Christmas Eve.
The four nation approach to all this has been disastrous, and must never be allowed to happen again.
Boris will put off a decision like that until as late as possible. He will probably kick the can down the road long enough for vaccinations to start having some impact (if Oxford roll out starts next month) and hopefully prevent it from happening. My money is on England and Scotland having tight measures until spring but avoiding national lockdowns. England only finished a 1 month lock down 15 days ago!