We're not talking about whether it "rules the land", AP. We're talking about what the predominant culture is. I would guess that, if asked, most people in the UK would suggest that a Christian culture is the one most prevalent in the country. This does not mean that most people go to church three times on Sunday, or that they have their children necessarily Christened or that they insist in marrying only in a church. It means that they believe that the cultures and values laid down by the Christian faith (when the church really did rule the land) are still extant today and that those values are predominant. Among those values, of course, are tolerance of other people’s faiths and beliefs but this should not mean that unreasonable accommodations should be made for them. I have no faith whatsoever (although I was Christened in a CofE church) but this is how I would answer the question.
By contrast in Muslim countries Islam really does rule the land. The faith is all encompassing and influences virtually every aspect of the followers’ lives (hence the original topic of this question). That is what a number of Muslims in the UK would like to see here and that is where the problems begin. The answer to that problem is to say firmly and clearly that those not interested in Islam will not be effected or influenced by its teachings, those wishing to follow it must do so without expecting unreasonable demands to be met and that should have been made clear two decades ago or more.