I have the Blue Light Card (well my wife does) and not everyone offers permanent NHS discounts.
Some of the businesses offer time-limited offers on certain things. For example, a phone company offers Blue Light Card holders a 15% discount off a newly launched phone/price plan.
If you do not want to do the whole permanent discounting because your profit margins are already very tight that is completely understandable, you are a business, not a charity after all.
However, if you have unsold stock you want to discount on a "When it's gone, it's gone" or you have a brand new product and wish to market it heavily, then the NHS discount for a time-limited period will get you a 1.4M (more on Blue Light as it covers Police and Fire too) audience of people. All you would have to do is factor in the discount on the price of that product for the time of the promotion.
Again, you don't have to do it, there will be very few people know or even care if you offer a discount or not, so don't be emotionally blackmailed into offering one if your business cannot afford to, but on the flip side, if you have to spend £XXX a year on advertising and marketing, then maybe consider a short time/limited stock discount as a way of tapping into millions of employed workers who may want to take up the offer. After all, offering a small discount might actually be cheaper to your profits than spending a fortune on advertising to 1.4M+ employed people. Particularly if you only offer it on one product rather than a flat rate discount on all products.