Honest but ill informed. The cheap helmets are tested to exactly the same standards as the expensive ones and probably have a bigger margin on the test limits than the expensive ones. What you are paying for on the expensive ones is the addition of ventilation holes which makes the design and production to meet the test standards more challenging. But better impact performance? No
There is no "standard" tetsing for helmets in the UK.
The requirement for sale is EN1078, but this is simply so weak that helmets meeting this standard are banned from use in the United States for racing.
There are a whole range that differ in the type of anvil tested ( some simulate kerbs, others don't) the level of impact tested and a whole range of other factors.
CPSC, ANSI, ASTM and Snell are some of teh standards.
The toughest of the lot is SNell who use "off the shelf" helmets, higher impacts and kerb shaped anvils.
A Snell certified helmet is proven to withstand greater impacts and more appropriate impacts than an EN1078
However the range of helmets passing Snell is limited in the UK, and mainly Specialized. Prices vary form about £30 - £169