I don't think that's exactly what LLoyd's did. I think they basically just said don't go through the straits right now or we won't insure you again. They only cancelled the policies of ships that aren't already in the area, which is legally allowed. Still a pretty blatant leveraging of their monopoly power, but they didn't renege on their contracts.
Reporting on this has been vague to the point where I’m starting to feel it’s deliberate to suppress the information as much as possible.
The most details I could easily find are from this source.
From midnight London time on March 5, war-risk covers from seven clubs will be terminated automatically if vessels enter the Persian Gulf, specific adjacent waters or Iranian waters, according to notices seen by Bloomberg News. The notices stated that the withdrawals are a result of reinsurers for such policies issuing similar cancellations.
…
Adjacent waters to the Persian Gulf include the Gulf of Oman, and waters west of Oman’s Cape al-Hadd, stretching northeast to near the Iran-Pakistani border.
So, my reading of this is that for ships already in the Gulf and trapped there, yes, technically you may still covered, but as soon as you try to leave the gulf, your coverage is automatically terminated.
That’s the equivalent of your car insurance being unilaterally changed by the insurer such that if a riot happens, you cannot leave the riot zone without your riot insurance being automatically cancelled, so now you are stuck in the hot zone hoping the rioters don’t decide to set you and your car on fire despite having bought additional coverages specifically against such riots.
But no matter how you cut it, western insurance cartels are predatory and massively abuse their market power as well as exert political power on western governments to write unjust laws to facilitate them ripping off their clients as much as possible. The whole industry is rotten to the core and the whole world would be far better off if they whole lot of them are replaced with a much fairer and just system designed to actually provide insurance instead of a means to extract unreasonable profits.