Britain is not a Christian country.
The religion of Christianity was imposed by early medieval rulers who wanted to join the urbane club of the European ruling classes, all of whom had jumped on the Christian bandwagon.
In subsequent centuries, majority attendance at church was enforced by fines, and in some cases, imprisonment.
The number of people who are interested in spirituality and religion is a minority. Among those who are interested, different models of how it works prevail; and fewer and fewer of them accept the Christian model.
Many of the values which are claimed to be Christian (compassion, forgiveness, love) are universal; and some values which are claimed to be Christian are either secular or come from another religious tradition (tolerance, inclusiveness).
Britain was originally a "pagan" country – that is to say, it had a number of indigenous autocthonic traditions. Now it is religiously and spiritually diverse.
As the recent survey by the Richard Dawkins Foundation for Reason and Science has shown, the majority of people who identify as Christian don't actually understand what they are identifying with. Now, I would like to see Christianity becoming more inclusive of different viewpoints, especially the view that other faiths are equally valid; but sadly, since the advent of fundamentalism, what it means to be a Christian has narrowed considerably.
In view of all of the above, I think that disestablishing the Church of England and reducing the role of Christianity in public life is desirable. However, I think there's nothing wrong with having contributions on spiritual and ethical matters by people from many different traditions (including humanism, naturalism and atheism) as part of public life, as long as lots of different traditions get access to the microphone, and not just Christianity.
Spirituality and religion are part of what it means to be human – but they are not exclusive to one religious tradition.