I'd make sure about helmet certifications first. Back in the day, my AGV rep wanted to give me a helmet to race with, but couldn't because the one he had didn't have DOT certification...only ECE. It was their best helmet, and stacked up well against the Shoei's of the day, IMHO, but the lack of DOT/Snell would've prevented me from ever using it, even on the street. WERA has since changed the rules to accept the ECE standard for the track; but if it hasn't been submitted to DOT, it's gonna be a problem for you.
This is a good point. It depends where you are buying from
In the UK it will have to have passed BS 6658:1985 standard.
stickers are:
Red sticker – Type A/FR: Used for helmets with a fire-resistant lining (normally used in car racing)
Blue sticker – Type A: Used for high-performance helmets offering a better level of protection
Green sticker – Type B: Used for lower-rated helmets.
All of which are higher than DOT standard.
From Europe it must be ECE22.05 or ECER22.05 which is the equivalent of DOT standard.
Bikes in the UK may also come with an ACU ( the UK equivalent of your AMA) gold standard sticker meaning it is suitable for racing. Those are the ones we buy. BS Standard type A ACU Gold. That's a higher standard test than DOT or SNELL.
It really should not matter. Helmets don't change per market. If you buy an RX7GP in the US it's the same as the one you buy in the UK. Only the tests it has to pass are different, ours being more rigorous. So it's down to the manufacture. If it's an obscure cheap helmet then stay away. If it's a shark, AVG, Arai etc that you know has passed DOT in the US then the same one from the UK will be exactly that, the same, except that it has been certified to more stringent parameters.
If you want to buy in Europe there are probably members here that can buy in store and post via a courier collection service that you arrange marking the item as a gift so as to avoid duty. That requires a certain level of trust because you have to pay up first.
If you would like to know the safety specifications of helmets sold in the UK then check here. Ours is a legal requirement, not an industry test.
SHARP Helmets - THE HELMET SAFETY SCHEME