First, a minor correction: there was no "England" before the introduction of Christianity.

As far as the common religion in Britain before Christianity, I guess this one depends on which introduction of Christianity we are talking about.

Before the Romans introduced Christianity into Britain, the dominant belief system was Celtic polytheism/paganism. This was the religion with the priestly class called the druids (who we have all heard so much about, but who we actually know very little about). Celtic polytheism is kind of an umbrella term for the beliefs of the 'Celts' (that can be a tricky term) which covered most of Western Europe from about 450 BCE to about 200 CE, depending on the particular location.

Christianity was ubiquitous in Britain (particularly in urban centers) by the 5th century.

This changed somewhat after Rome withdrew from Britain and the Anglo-Saxons moved in. The Anglo-Saxons were Germanic pagans who only started adopting Christianity in large numbers in 597 CE. So for about 150 years the Anglo-Saxon controlled portions of Britain (the south and east coasts - Sussex, Kent, Essex, East Anglia, etc.) were likely made up of a mixture of Germanic pagan Anglo-Saxons, Celtic Christians and, possibly in some deep rural areas, Celtic pagans.