Anything to be sold in the UK must pass the EN (European Norm) standard, which is the euro equivalent of a BS.
Helmets may actually perform better than this, but are not tested beyond this. Having said that, the same helmets are sold the world over so any helmet from Giro etc will / should also conform to the US standard, which is believe is slightly more rigorous.
In practice the testing involves controlled impacts on helmets pre selected by the manufacturer, except for Snell certification, where the tester selects actually helmets on sale. However, it costs a few cents to get Snell certification and it doesn’t seem to help sales as few people take any notice of the certification that comes with a helmet, so most manufacturers don’t bother. I think only Specialized do. This doesn’t necessarily mean that Specialized helmets are safer.
As far as the second part of your question is concerned, about evidence for their use ... the search function will bring up pages of arguments.
Intuitively one might expect that helmets offer some protection, but it is limited at higher speeds due tot he increase in energy involved in the impact (increases with the square of speed - i.e. twice the speed is four times the impact energy). there is also a suspicion that helmets can impart rotational forces if they skid along the ground (especially for helmets with pronounced rear vents that looks like a ducks bum).
It is certainly true that the act of cycling confers more health benefits than the risk of cycling without a lid. So if wearing a helmet puts someone off cycling then they would be better off binning the lid.
I personally used to find helmets unpleasantly hot and sweaty. I now have a Giro Atmos, which is okay, and I wear it in the hope, rather than expectation, that it will protect me if I have a crash. Unfortunately the Atmos is quite expensive. It is almost certainly no safer than cheaper helmets - probably less safe due to the large venting area - but at least it is wearable.
