How do they count viewer numbers? With many houses having multiple occupants, there will be many cases of multiple people watching one television, so does 14 million mean 14 million people, or 14 million households? If the latter, the true number of individual people watching it will easily be well above 14 million. The UK population covers babies/young children/those without access to a TV, so really you want to compare number of people watching to number of people who have the ability to voluntarily watch it.
I was under the impression that viewer numbers were mostly calculated by using a representative panel and then scaling that up. The panel report how many people are watching each device in each household, which can then be used with the statistics they get from the streaming platforms.