The nhs ones are unnoticeable from the front and my hair is very short. I do keep a bit more hair behind the ear to just curl over the hearing aid.
In my opinion, looking at friends, the ‘in the ear’ is much more noticeable. It sits there like a little plug because it has to be big enough for a battery, however small. The little receiver in ‘behind the ear’ goes right into the ear and the wire coming out is thin clear plastic that blends in with your skin.
As far as irritation and infection you do have to experiment with end pieces, both size and material. I had some problems at first because my ear canals were different sizes so the first ones were too big on one side. You can be allergic to some plastics so need to get what suits you. But this would be the same whether it was private or NHS, in the ear or behind the ear.