Underfloor Heating | Electric | Warm Water | Floor Insulation | Frost Protection | Uheat sell it you could ask them for a bit of a small stock for yourself, and then mention it to a few other tilers, and perhaps even make back your delivery costs by selling it for a slight profit to cover it perhaps?
If you can buy a fair pallet load and shift it regularly, Marmox will even give you an account for you to buy direct, I'm sure. Worth considering if it's so rare, maybe?
Not sure what the score is with transit from the UK to NI to be honest, whether ship or air is used or what. So not sure what would work out cheapest but worth considering because the amount of jobs it can be used on is quite high and people tend to cut corners when they're not using such products.
I think Wedi is quite heavy so you'd have to factor that in when considering shipping with that stuff. But Marmox is like transporting air in a box around. It's just a big box.