You can always go straight to FreeMBD, and bypass Ancestry.co.uk.
They are at
http://freebmd.rootsweb.com/This site is completely free and has some features I've not found on Ancestry.
They are also running FreeREG (parish register transcriptions) and FreeCEN (their own transcriptions of the censuses) but it's early days yet for these two projects.
For example, you know that John Smith married Mary ?
If you find John's marriage On FreeBMD you can then see all the people on that page of the register (NOT the index), usually only 4, two men and two women.
One will be Mary Jones and the other Joan Blogs.
Now you've found Mary's maiden name and the registry entry, you can get the Marriage cert. and find their fathers, and so on.
You don't even have to know Mary's name at all. Look up the other man in the next census and find his wife's name (Joan), then by a process of elimination, the other woman is John's wife. Doesn't always work, but well worth a try.
Not so easy to do on Ancestry as it only seems to have the index, dozens of people on one page, not always with their spouse's name.
I could be wrong on this but I've not found the data in the same format as FreeBMD.