Motorcycle and Scooter appreciation

Go to the biggest shop you can find. Try on as many as possible. Once you find one you think fits your head and budget, keep it on and wear it while you try on gloves. I've probably ended up with a too small helmet because I didn't spend enough time with it on.

My personal unscientific feelings are:

• £100-250 buys you a lot of helmet nowadays, HSJ, Shark, caberg for eg are decent and inexpensive.
  
• internal visors are really useful (and saves you buying spare visors)
  
• pinlock compatible is a must (something that includes free pinlock insert even better)
  
• white helmets are more visible and match as many colours as black
  
• once you buy check if you need to register it to validate your warranty - AVG for eg give a 5yr warranty if registered vs 2yrs without.
  
  
  

If you're on a tight budget I'd stick to helmets with less features. In particular fixed/non-flip up ones and old fashioned D-rings. That said if you're in the city or do deliveries then it's quite useful. The Sharp's helmet rating website gives lots of detail on safety - including flip up failure rate.
https://sharp.dft.gov.uk/

Ultimately helmets are a pain. The only way to really test them is to buy and ride for a month.

Edit: if you have a search of this thread there's some useful pointers.

What kind of riding are you going to do?

As mentioned try on as many as possible, I find Arai the best for my head shape but have used shoei and hjc with no issues. I've had flip up ones for work and actually got a cheap OP one now TBH unless you need to chat to people a lot I would not bother with one unless they're a real bargain. The internal dark visors are well worth getting though. At the risk of sounding like an old bugger, if you're new to biking when you get your helmet get ear plugs as well even for commuting.