Just spoke with a friend of mine who's a travel agent specialising in Cuba. He says that you may well have your insurance checked at Immigration when you arrive as the Cuban health authorities can't get paid by US companies in the event of somebody being taken ill.
If Insure and Go UK guarantee payment for Cuba, no problem, but otherwise you may be required to buy extra cover there and then. This is, apparently, a recent requirement. Interesting.
Might be worth while getting something in writing from I and G, or taking up Thomas Cook,s no doubt generous offer.