In reality no a discounted tub of formula in a UK supermarket is unlikely to do damage either to breastfeeding rates or formula fed babies.
However the code is international and exists for the protection of all babies. It has to cover even things which are mostly harmless such as this because that way they can block much more insidious marketing techniques.
Techniques like sales reps who dress in similar costume to HCP uniforms and market formula in hospitals and clinics to new mothers as a "scientifically better solution". Seems unbelievable but used to happen here. Techniques like delivering free samples of formula to mothers, whole cans, so that when you're struggling in the first few weeks of BF it's just handily there in the cupboard. This happens today in the US.
Techniques like offering free samples of "scientific" formula to mothers in developing countries with very low levels of education and literacy, giving them enough for their milk supply to dry up and then charging them full price and profiting while babies starve. The mothers can't afford the formula so they use dried whole milk or water down the formula to eke it out. This has happened. It still happens.
A 5% discount on a full price tin of formula in Tesco is not a problem but these other marketing techniques and things like them are definitely a problem. The issue is you can't just say well ban all of those practices but not this one because this one is definitely not harmful and I like it. It's easier and more effective to prohibit all of them in order to prevent the worst offenders. Even the free formula tin samples in the US are just an extended version of a discount.
If you block some practices but not others the companies find ways around and loopholes in the law. Another issue is that the line is often crossed anyway. When the line is drawn right next to practices which are harmful you end up with harmful breaches occurring. Currently we get plenty of breaches but they are all in the harmless zone so it doesn't matter too much, but it does mean that we need to uphold them. It happens fairly regularly with TV ads - a company will make a spurious claim or add a slogan which it is then later asked to retract, and it does, but the image has still stuck with people. If I asked you which formula is closest to breastmilk, you all know which formula company claimed that - that was illegal, because no formula is closer to breastmilk than any other, yet the message persists. If I asked you which formula provides as much calcium as a giant comedy beaker of cow's milk, you know which brand I'm talking about - that advert was illegal too, as it contained misleading information.