There's always the hard slog of 1837online. If you have, say, a 10-yr-old child in the 1901 census, you can search for marriages from 1910-1920. This works well unless the surname is common. At this point you need to order the certificate to get the other spouse's name (which also serves to confirm you've got the right marriage, through the father's name). Then, thanks to the introduction of mother's maiden names from July 1911 (or was it 1912?) it's quite easy to trrace births. Once you have a birth with the correct surname and mother's maiden name, say in 1915, you can search for marriages in 1935-45 ... and so on.
I've done a couple of generations this way.
Cheers
Tim