On return your ex and your children need to quarantine for 14 days AWAY from you (and everyone else) so they can't stay with you.
Exactly. This should be your answer to him.
This quarantine thing, as implemented, won't work to prevent transmission, btw. It's a tactic by the government to discourage people from travelling.
But I wonder what they'll say at the airport when he tells them that his plan is to take the children to their mother, thereby flaunting the quarantine.
If they go and he attempts to drop them off, then you can certainly refuse to take them and if necessary call the police for breaching quarantine. Tell him that you WILL do that.
Honestly, a parent who is prepared to put their children through any of this, is a bad and selfish parent.
He could have chosen a UK holiday, instead. Or just go alone. Or accept the cost of his decisions.