Grouch.....I may be able to help you....we used to own a motorcycle clothing and accessory shop on Wirral and have some helmets left over. They are brand new but would need to know a few more details from you.
The most important thing about a helmet is the fit......end of!
The best way to look at the cost of a helmet is to use a car analogy......if you needed to travel from Land's End to John O'Groats and had the choice of two cars, a bog standard Fiesta or say a top of the range Merc which one would you choose? They will both get you there but clearly one will get you there in a greater degree of comfort.....adversely if you you only had a 5 mile journey to make each way why would you need the Merc because you wouldnt be in it long enough for its creature comforts to make a difference........
A £50 lid will do the job provided it is accredited but the more expensive will offer a more a greater level of comfort......but......unless it fits properly it doesnt matter what you pay for it it wont do the job of saving your life if it needs too