Half the people may be non religious...but are they also non spiritual? That is important to know! Many people could reject religion (mainly Christianity I guess) and take up secular forms of spirituality.
Secondly, the UK being very small, its growing atheism (even if) need not be of any significance because many large countries like India, America have increasing numbers following either some religion or some form of secular spirituality. Even in countries like China and Russia where people were largely atheistic, they are now becoming more religious/spiritual.