Britain was Pagan long before it was converted to Christianity. Many Christian customs, traditons and feasts are old Pagan ones that have been modified to suit Christianity and that includes Christmas (Yule) and Easter (Ostara). Paganism is an umbrella term for all religions that are not one of the three Abrahamic religions, that is to say not Judaism, Christianity or Islamic.
Other religions such as Budism, Hindu, Taoism, Shinto, Wicca, Odinism, Druidism and many others are all considered to be pagan.