That's an interesting answer.
I wonder if you could perhaps ask them the same question, this time casually mentioning a U.S. Experimental Category aircraft
An whole lot of people, myself included, would be delighted to sponsor the cost of typing up the letter.
But seriously, I am not suprised by their answer. The trick will be to check if the same aircraft would be legal to fly in the USA.
And obviously make a full disclosure to the insurance company. That's what
really counts in these "everybody knows this is safe but is it legal" situations. The "most" the CAA can do is prosecute you.... other outcomes are much worse.