The whole things seems to be a bit of a hollow victory for these campaigners.
How is The Sun any different from magazines like Zoo in that they're privately owned institutions who offer the opportunity for women to model in exchange for money? They're objectively absolutely no different, both of them can decide to put what they want within the pages (within reason) and then the consumer can decide whether they want to buy it or not. If you don't like them putting women on page 3, don't purchase it, or gain public support and boycott it.
You're either against ALL objectification of women in magazines/newspapers (which seems a bit Victorian and many would argue actually restricts the freedom of women to choose what they want to do with their body) or you just try to avoid purchasing the material of a specific product.
The only reason they've targeted this one example is because it's got a higher level of circulation. It feels like it would have made more sense for them to target the industry and ask for literature containing nudity to have a minimum age of purchase, maybe? Not that I particularly agree with that, it's more that I don't particularly understand what this has really achieved in a wider context.