Most helmets on sale in the US will be to DOT and / or Snell spec. They may well not have ECE 22.05 certification unless they're European imports and will therefore be illegal for use in the UK.
Often quoted and complete and utter rubbish !!!!!!!
It is illegal for any supplier to SELL a helmet that is not ECE 22.05 rated however for the law you have to wear a helmet that is manufactured to a standard that would give equal protection in event of an accident, and that applies to the quality of the shell not the ammount of shell so those stupid beanie helmets that have passed the snell test are legal to use because the shell that is there is made to an equivilent standard despite not passing the EU test because there is not enough of it.
From a legal standpoint anything that has passed the Snell test or DOT testing in the US would be legal to wear just not to SELL as a retailer, you can happily sell it as a private seller.
Notes
(i) Every person driving or riding on a motor bicycle (other than in a sidecar) on a road -
(a) must wear protective headgear, which
(b) must be securely fastened to the head of the wearer by means of straps or other fastening provided for that purpose (if it has a chin cup it must have an additional strap to go under the jaw), and
(c) must bear a mark indication in compliance with the British Standard/equivalent EU standard, or
(d) be of a type which, by virtue of its shape, material and construction could reasonably be expected to afford protection similar to, or greater than a helmet which conforms to the latest British Standard 6658:1985 (or equivalent EU standard).
(ii) In (c) above, the British standards Kitemarks which are acceptable are B.S.2001 :1956; or 1869:1960; or 2495:1960; or 2001:1972; or 5361:1976; or 2495:1977 or 6658:1985 (per Schedule 2 of the Regulations). Any of these will do, it does not have to be the latest Standard. The Standards are amended over the years (the 1960 Standard has had 8 amendments, the latest in 1975), the helmet does not have to have the latest amendment.
It will be for the prosecution to prove that the driver or rider of a motor cycle on a road was not wearing protective headgear of the appropriate type for a prosecution to be brought.