Right this has caused a bit of a debate at my place of work - but here is the short version
Technically yes you would be committing a offence under section 16 of the road traffic act 1988
Here is what it states in the act. now i am not going to type on here the full details of the above act, but it does state that in paragraph c
"
Must bear a mark indication in compliance with British standard/equivalent EU standard"
But if you were "visiting" this country, you would not be committing an offence as you would be here on a temporary basis, lets face it, if you go off touring around the world you do not need to take a helmet for each diffierent country you visit. no
As for the visor - that will also have a bear a BS or Eu standard number and bear on it "for daytime use only" - failing that do as i do and carry a clear visor with you - a pain i know but might save you a few points or a fine
Now ive have been lucky (or should that be unlucky

) enough to of only been stopped twice so far, and neither time have they checked my cash helmet, so unless you get stopped at a department of transport check or have been stopped by PC Grumpy on a really really bad day then it is upto you - do you you take the risk
i feel like i should put some sort of disclaimer in this post

so - use the above advice, comments at your own risk
Hope this helps - Adrian