What happens if, under new UK rules, my employer wants to claw back a bonus that I’ve spent and I don't have any savings? Could I be forced to sell my possessions?

It's hard to avoid speculation at this point, since new rules were only announced by the Prudential Regulation Authority this week. Given that nothing is coming into force until next year - meaning that it will be a while yet before we see any such clawbacks in action - we can't predict precisely how a clawback of a vested bonus could be enforced.