Help on HGV licences/categories

I need to shed some light on HGV licences.
Basically ive taken the route of doing my HGV to drive horses boxes in the UK.
I live on the Isle of Man and have my car licence. Which im hoping to add the C1 category to this to drive horses.
Our licencing is the same here as the UK and never had any problems driving in the uk for some time on my Isle of Man licence.
However I have been told its not all the same for the HGV.
And that i would have to take the UK version? Even if i had passed the test on the island.

As far as i knew the only thing i would need to drive horse boxes is the type 1 and type 2 competence certificate for transporting animals- applications on the DEFRA website. Along with passing the correct test i want to drive on.

Im trying to do a bit more digging after being told its not all the same. If anyone has any idea i would really appreciate your help.
Thank you