I think you are probably wrong to use only your .ac.uk address in order to secure the discount if you are no longer connected to that institution.
However, if companies are allowing student discount based on a ac.uk address that is their mistake as anyone who works at universities has these addresses not just students.
However, I think it is perfectly legitimate to use a lifetime NUS card to obtain a discount for NUS card holders even if you are a relatively well paid lecturer as you meet the conditions of the discount. It's to attrack custom and not just aimed at the lower paid. Similarly NHS/emergency services workers can get a whole host of discounts, despite some earning above average wages.
Many goods and services are priced now so that a certain percentage of purchases will be discounted and discounts are easy to find - so in a way, you are being taken for a mug if you pay the inflated full price.
There are certain things that I would never pay 'full price' if I could avoid it, such as:
Pringles - never pay more than about £1.20
Chain restaurants - Total rip off without a voucher
Wine that is regularly sold at 'half price' of around a fiver - never worth the claimed full price.
Most theme park type places - easy to get a 2 for 1, or free child entry
Christmas gifts - many are still a rip off in the 3 for 2 deal
Always search for a discount code and cashback deal before buying online.
Gas/electric/phone/broadband/TV etc - move regularly to get the introductory deals - pay full price they are taking you for a mug.
So if you weren't getting the student discount, it probably wouldn't be hard to find another discount for the same items.
However, I hate having to do this, it's exhausting - why they can't have more consistent pricing and reward loyalty, I don't know. It's one reason why I prefer Aldi to the major supermarkets. As well as a whole host of other benefits, the pricing is consistent and you don't have to chase often fake half price or two for one deals to avoid being ripped off.