"Who do you think you are" is a load of rubbish on how to do research. What people are shown is interesting sometimes but a lot of work went into finding the data.
UK death certificates are not on line - only the index from 1837 onwards is - from mid 1866 it shows age at death. So can be very difficult to know when you have the person you want
And also alas when you buy the cert it can be difficult to verify the person - such as they die in medical institution and informant is also medical
BUT some are wonderful sources of information
When you have the reference for the cert you want
buy the paper copy from http://www.gro.gov.uk/gro/content/
Barbara has some good Help on BMD http://www.dixons.clara.co.uk/Certificates/indexbd.htm
includes what is on certificates