The UK/EU version of Oxy-Powder is exactly the same as the US Formula - there is no difference. The UK and US Oxy Powder is manufactured at the same facility. Both products are the same strength, and taking an equal dosage of either will produce the same results.
The reason for confusion is the due to the fact that EU labelling laws require that the UK label is different to the US, hence the different label and description, as well as dose. In the US, GHC are able to list the magnesium salt compound as the ingredient but for the UK/EU they are required to break down the minerals oxide and hydroxide. Differences in regulation are also why they are able to have "Gluten Free" on the US label but not in the UK. In the US they are able to label a product as Gluten Free if the ingredients themselves are gluten free and no gluten is processed in the same facility. However, for the UK it is required that the facility itself is certified "gluten-free" in order to advertise it as such which is why the UK bottle is missing this symbol.
Another difference in the label is that the US version of Oxy Powder states the ingredients per dose of 4 capsules whereas the EU/UK version states it per capsule. The US serving size is 2746mg (for 4 capsules), whereas the EU/UK version is labelled as 687mg per capsule. You would need to multiply the UK version by 4 to have a like for like comparison = 2748mg.