If a helmet does not fit and is uncomfortable, then it is a rod for your own back
The first point is that it should fit and be comfortable.
Research shows that an ill fitting helmet can INCREASE injury two fold when compared with a properly fitted one
After fit and comfort comes the safety standard
EN1078 which is required for sale in the uK is inadequate and feeble.
Australia, The US, and many UK organisations do not allow their use!
The "toughest" standard is the Snell B95, followed by the Snell B90.
ANSI, CPSC or ASTM test passes are all superior to EN1078
Cost is not really an issue. A Snell rated helmet can be sourced for as little as £30 - price is NOT and indicator for quality
As above - visit a few shops as helmets are like shoes. You may find one manufacturer narrow, and another too wide....... trying them on is the only way to find out