How much to pay for a cycle helmet?

I have had my Raleigh road bike for around ten years and during the summer,
I use it once a week and hardly ever during the winter. So far, I haven't
got around to getting a helmet. After some recent road accidents involving
a couple of my colleagues I think it's about time I bought one.

I have seen several helmets being sold for £20 or less. Does it make any
sense to spend a significant amount of money on a helmet? I realise it's
my head which is at stake but how much is it worth paying? I know that
some will say that considering there isn't much in my head, then it's not
worth spending very much. But I do value my head very much!

Are there any safety standards which the helmets have to meet for them to
be on sale in the UK or can anyone make an untested helmet and then claim
that is it suitable for use by cyclists?

Are there any independent tests which have evaluated the merits of
different makes of helmet?

Thanks

Mr Benn