They are probably wanting to steer you to a private sale as they will, I speculate, make considerably more money than dispensing an NHS aid. There should be a range of NHS aids, so you need to make is clear your needs (Bluetooth for me is vital to enable me to work online).
There should be other NHS providers in your area, so you may be able to shop around. There are two in my local town, one being Specsavers (but only for over 55’s, so no good for me).