Just to expand on the earlier replies:
When my hearing started to deteriorate I first of all had a hearing test carried out by a nurse at my GP's surgery. The nurse explained that this test was relatively crude but would give an indication of the extent of my hearing loss. After the test, my GP looked at the results and said that although the test was pretty basic (not done in a sound-proof room, etc) it was clear that I would benefit from hearing aids. I mentioned "going private", as I usually do when faced with any specialist treatment. His reply was that he had no objection to private medicine or surgery in most cases (and indeed he has happily given me referral letters on a number of occasions) but in the case of hearing correction he was firmly of the opinion that the hearing aids provided by the NHS were not only perfectly adequate for mild to moderate hearing loss but were also a lot easier to get used to than some of the fancy "invisible" hearing aids sold at vastly inflated prices by the High Street opticians. His suggestion was that I should get a proper NHS hearing test and get some of their hearing aids. After getting used to these, if I felt I required anything more fancy, I could always at that stage go to the commercial enterprises. I followed his advice and have now had my NHS hearing aids for just over 3 years and I have no desire (or need) for anything any more sophisticated.
By contrast, my sister in law, when her hearing started to deteriorate, immeditely went to one of the High Street opticians and was sold a pair of hearing aids costing almost £1000. These are very small, so small in fact that she has difficulty putting them in and because she finds them uncomfortable she only wears one of the pair in her "better" ear. She feels that she has been conned out of £1000 but is in some sort of denial that NHS aids would be any better as she is a firm believer that "private is best" and if her expensive private hearing aids don't work there's no chance that free NHS ones will be any good. Sadly, she won't even try my recommendation.