I don't think you'll ever see a 'cave in' per say. as it would be a political non starter. Most likely is that the UK will go into a transitional relationship with the EU where it de facto adopts something akin to the Norwegian model and essentially never leaves it, citing that conditions would not be right to leave completely until all negotiations are finished, yadda-yadda...

I would keep my eyes open to both sides softening on the finite nature of the present transition plan as a sign of this becoming more likely.