I think it very much depends on the budget of the NHS trust, though...
My DS2 has been severely deaf since birth, and gets the latest model Phonak aids, updated every few years, through the NHS - he's seen at a central London hospital due to his v rare genetic condition.
I have a mild/moderate loss, and finally took the plunge with aids a little while ago. My local South London NHS trust would only give me 1 aid (despite my hearing loss being the same in both ears). It was not a Phonak. The size of it was about twice that of my sons (despite my hearing loss being much less severe
) - it was massive. And it had no Bluetooth functionality, no programs I could change between in different situations etc. I asked whether this model was the one they gave out to everyone, and was told yes it was.
So I went private, and now have the same aids as my son. The hearing test results I had done privately were the same as my NHS results - I compared the two. The aids cost me £3k, but I would rather spend the money than have the frankly crap aid that the NHS gave me! (I realise I am fortunate to be in this position).
So to all the PPs saying just go NHS, you get the same or better aids than private - it's definitely not always the case! Yes, certainly try that route first, you'd be foolish not to, but don't assume that what you get from the NHS is best in class.