I have no information for you on the UK site. But be aware that the prices you see are in Euro's not U.S. Dollars.
I find the sizing very similar to Shoei or HJC. FYI - It is possible to find a local dealer that sells the Nolan "Trilogy" and try that helmet on for size and comfort. It is the same helmet without the "AIR" top venting.
Look again. Those prices are British Pounds, not Euros.