A useful link that (unlike some other posts here) actually seeks to answer the question:
http://www.paulwilliamsfunerals.co.uk/funeral-services/religious-funeral-services/jehovahs-witness-funerals
As has been suggested above, anyone living within a Church of England parish has the legal right to be buried in the parish churchyard (which may or may not be alongside the church) irrespective of their religious beliefs (or lack of them). However a Witness family might not be comfortable with such a burial, so a cremation is probably more likely.
The question asked here (along with quite a few others that have appeared on this site in the past) seems to assume that burial of the dead is the norm in the UK. It most definitely is not. Three quarters of all deaths in 2012 (74.28% for the pedants out there) were followed by cremation.