Ancestry has Birmingham records, for example, while neighbouring Staffordshire records are on Findmypast.
It helps to know which parts of the UK you are most likely to be researching as Ancestry and Findmypast in particular have exclusive relationships with specific archives and local authorities. This usually means that images to crucial parish registers, as well as other records such as land tax, prison and quarter session records may only be available from a website that has a relationship with that specific local authority. This can sometimes be problematic. Ancestry has Birmingham records, for example, while neighbouring Staffordshire records are on Findmypast.