Researching cycling helmet standards is a real rabbit hole of acronyms, codes and bewildering language. To add to the confusion, not all countries adhere to the same standards.
In the US, it’s the Snell rating; Australia and New Zealand have their own standards with the most demanding testing; Japan has its own; while the EU has the CEN standard. Where the UK will be post-Brexit, who knows. You’ve then got additional ratings, such as MIPS (Multi-directional Impact Protection System), which, although not compulsory for a helmet to be rated for cycling, offers increased levels of protection.
So, where does this leave you? The simplest advice we can offer is to stick to well-known and established brands and, most importantly, try before you buy to find a helmet that fits correctly. Even the highest-rated helmet, if it’s perched jauntily on the back of your head or doesn’t fit securely, isn’t going to do its job if you have an off.