This is normal, they call it free market
It's a little ridiculous because it is from the same seller, but the weight of the unit can result in higher shipping costs, or maybe in the UK there are easily available alternatives, and not so in Belgium, so they decided to keep the price lower.
My personal opinion is that with 350 Euros you can buy something better, or even have it built to order.
I paid 350 Euros for a 230 to 117 V - 5 kV A
autotransformer built to order in an IP67 box , used for giving power to a boat built in the USA when it was standing in an Italian port.
Look for some transformer manufacturer near you, and call: maybe they have something suitable on hand.
Out of topic, but not too much: have you ever looked at IKEA's prices in different countries? You can find them on the net: sometimes the price for a piece of furniture is higher in the country where that item is manufactured....
Best regards