I've had both NHS ones and privateones – the private ones are carbon fibre and were about £200 i think. The NHS are crap by comparisom and only really where them in Wellies and heavy work boots – i have tried all sorts of off the shelf products and they are ok for occasional use but not for all day walking.
To be fair the NHS ones got better each time i went back – once the guy was convinced i would actually wear them – the latest ones are much slimmer and you can actually wear them in a normal pair of shoes.
The big difference was that when i had them made privately they took full on plaster casts of my feet so the fit was perfect – wheras the NHS just did it my eye really.
You pays your money………..