Third party bodily injury and third party property damage remains a mandatory insurance requirement across Europe including U.K.. Hence as said previously the Green card, which we had many years ago, is proof that car and hitched caravan are Insured for the mandatory minimum.
Where the caravan is unhitched , and rolls away as Lutz describes, I haven’t as yet seen a mandatory requirement for insurance in that scenario. My own CAMC policy does give the appropriate liability cover. I imagine others will to but check.
The concern will be those who do not insure their caravan separately or those who do not understand the subtle difference.
In fact prior to Brexit a lot of U.K. certificates of motor insurance had on the reverse side Eu words, French etc confirming cover just like the Green card!