How on earth would you go about sourcing MGO SIPS in the U.K ?

Import them from China... like everyone else does. But, you might want to just import the MGO (MOC) sheets, since it is pretty ludicrous to fill a container with 70% EPS, and then ship it 3000 miles. Even better to just import the dead-burned MgO, and find a local source for MgCl2. Salt lakes (like the Great Salt Lake here in the US) are perhaps the best source of aqueous magnesium chloride, and according to some reports, makes the best MOC binder.

Valid point concerning the the transportation of air. As the UK isn't known for it's high salinity bodies of water, would the sea do ? We have a fair proportion of coastline. How large a bucket would I need ?

MgCl2 is very readily available, but the ocean is only about 0.5% by weight. What you normally get is MgCl2.6H2O (magnesium chloride hexahydrate) - it appears as a dry crystal, but it actually has 6 water molecules bonded to it. It is pretty cheap, too - which is why it's used to de-ice roads. Current pricing for 95% is around $250 - $300 US / metric ton. And, any relatively pure (probably 80% or higher) material should work.

Some researchers though indicate that brines from salt lakes produce MOC that is more water resistant. In that case, it isn't (virtually) dry flake, but is delivered as a liquid.