Hi, I don't have a casco myself but I've just recently had a look at helmet safety standards, and I believe the toughest standard to pass is that of the Snell Memorial Foundation.
website here
The only helmets sold in the uk which pass this are made by Specialized, in fact I think all Specialized helmets have a Snell certificate which includes some of the more reasonably-priced ones. There's a list of certificated helmets on the website.
Obviously I don't know what floats your boat, helmet-wise, but if you're interested in the best protection these might be worth a look.