I'd hazard a guess if the UK vehicle has a SORN it means it is off the road and therefore may not even have a valid MOT, are you sure it's such a great deal? Perhaps this is the reason with insurance company is dragging it's heals. Personally I have never had a problem in arranging cover either way, that is UK reg for for Ireland and IRL to UK as I have had to do both at different times.
I guess if you ask exactly what is the problem from the insurer they will tell you and then you may sort it out. I actually never heard of them being difficult in swapping insurance before. I one had to swap 4 times in one week!