What that implies to me is that the church will use those numbers to say, "Look, we support this and we have X million followers! It's popular!"--trying to emphasize how many people they have who allegedly support the legislation. Not that they have greater representation in the legal system because of it, but that they can put more pressure on the representatives who are there. If people leave, that decreases the leverage the church has because X-minus-1 million is not as impressive as X million.
But then I'm looking at this from an American standpoint (particularly the word "lobbies", which to me is something government is on the receiving end of rather than a function of the government itself), and I recognize that all this may be completely different in Britain. I know nothing of the way the British legal system works, I'm afraid.
What you wrote above is different than the initial post where "alloted power based upon the number of followers." The paragraph above is very much in line with the US. Various minority groups advocates get very worked up over each US Census. Aside from the number of people in each district, it helps with lobbying for money, projects etc that will particularly benifit a given group.
For example, in Lebanon, the Sunni Muslims get a certain number of seats in parliment; Shia get a certain number, Orthodox Christians, etc. So if you are a Sunni, you are runing for a seat that by the constition can only be held be a Sunni. The number of seats allocated for each religion was initially based upon the 1932 census.