Not sure where you’re getting your pricing info, but when I go to amazon.co.uk I find the price is £250. I know Amazon’s prices vary over time, but I didn’t think they did so wildly.
When you find an absurdly priced book Amazon, it’s often worthwhile to check the price at the company’s other national sites, where it often can be found for far less, even when you add in shipping and duty. As a general rule, I find the prices at the US site tend to be lower, but it varies with individual books.
Currently at the US site Green’s is US $650 (£410). At the Canadian site it is CDN $470 (£300). So it looks like right now, amazon.co.uk is the place to buy it, but for us Canadians the additional shipping cost from the UK would wipe out the savings. (The used edition I found, which was essentially new, was US $325.)