I was experiencing similar but managed to get my money back. I needed to force their hand with numerous emails. Got a full refund instead of a credit mine was a cruise but same principle applies if booked through a travel company.
While cruise lines' terms and conditions typically say they can change itineraries at will without recompense, the Package Travel Regulations overrule such clauses when changes are significant – such as changing the country of departure or arrival.
Yet for anyone who booked the trip under the protection of the Package Travel Regulations 2018 – through a travel agent or direct with the UK sales operation of the cruise line –*the law is clear: their planned trips have changed significantly and they are entitled to all their money back.
But the Package Travel Regulations*make it clear: “The traveller may, within a reasonable period specified by the organiser accept the proposed changes; or terminate the contract without paying a termination fee.”
If the customer does not wish to make the revised journey, they are entitled to receive a full refund – within two weeks
You are 100% correct re the Package Travel Regulations and glad you got your refund - the trouble is most Tour Operators are currently ignoring the Regulations. ABTA are being really unhelpful imho, because the law is the law and retrospectively changing it is never a great idea, hence why for the last 3 weeks despite ABTA saying that the Government will update re the situation, they haven't as I'm convinced the Government are nowhere near agreeing to it. It can be found here BUT this is "Advice" and not Law: https://www.abta.com/news/coronavirus-outbreak