Either Fridge Brilliance or Fridge Horror depending on your viewpoint, but at the end of 15 Million Merits, the main character, Bing, is seen looking out of what might be a huge fake vista of woodland, or is alternatively looking out of a real floor to ceiling set of windows showing lush green forest as far as the eye can see. This means that he has either swapped his tiny cell for a larger Gilded Cage or the endless drudgery of cycling that the majority of the populace does has enabled most of the planet to resort back to natural forest. Either ending can be seen as ultra depressing, but YMMV.
In 15 Million Merits, the citizens all sleep in their own single cells and are bombarded with pornography every few minutes. One method of population control...
Fridge Brilliance in 15 Million Merits: It's implied, when Abi goes for her Hot Shot audition, that the Cuppliance is responsible for her going into pornography more than anything else. But then, Bing goes on as well, avoids drinking it... and then goes along with the panel's offer and gets swallowed up by the machine anyway. Maybe the drink didn't really do that much, they just thought it did?
He did mean what he was saying: it was largely unplanned, and he countered one of the judges when he described it as an "act". The problem was that he had no plan beyond saying it, and the judges were "swayed", but not in the way he was expecting. They then decide to let him say all that stuff and even put it in a timeslot, while the crowd goads him into doing it. Regardless of whether it was true or not, or was a genuine tirade against the entertainment industry, a simple rant isn't going to change the system, but it's more likely the system will assimilate the person doing it.Bing clearly feels as unfulfilled in his new role as he was on the bikes. That's what the scenes were meant to illustrate.
He clearly was hoping to reach the judges and the audience and create a big emotional upheaval, which did not happen. So perhaps he figured that even though he would be absorbed into the system and reduced to one more form of pop culture, maybe he could reach at least one person, and be at least that tiny degree of successful communicating his message. One could also view it as the opposite that he was so disillusioned by the failure of his message, and the system's ability to subsume even the most effective criticism, that he lost the will to fight anymore, and decided that if he was going to give up, he might as well give up in comfort.
15 Million Merits? Why not 12 million? 20 million? Because in the future, everybody will be famous for fifteen minutes. May be accidental because the price used to be 12 million. The fact that Bing was forced to cough up the extra 3 million was a minor plot point to emphasize his self-sacrifice.
The usage of Giuseppe Verdi's "Dies irae" —— Day of Wrath —— as Hot Shot's theme fits in well with what Judge Wraith does, inflicting wrath upon contestants. It becomes more clear on what he did to Abi, turning her into a porn star, which is a wrath for her.
Bing's name. Like the search engine that got its name specifically because it's not google, Google potentially being one of the soul-sucking modern tech corporations that Brooker's message is always aimed at.
Fridge Horror after watching Black Museum — a character in that episode reads a 15 Million Merits graphic novel, suggesting that the story in this episode is being used as straight-laced entertainment in the future (compared to the "this is bad" entertainment it was in Real Life).