Might be hard to prove, but it could be your William Edward Webb who married Elizabeth Harriet Coleman in Reading in 1874. Berkshire BMD:
http://www.berkshirebmd.org.uk/shows this marriage as taking place at the register office, so unfortunately the only way to see it would be to buy the certificate.
There is a 19 year old Elizabeth Coleman living in Reading in 1871, along with her widowed father William Coleman. She was born in Reading.
A search of the 1881 census shows an Elizabeth H Webb aged 29 born in Reading. She says she is single and is a servant in a household in Reading.
There is no Elizabeth Harriet birth of any surname registered in Reading in 1851/52, which muddies the waters a bit at this point. There is an Elizabeth Coleman registered in 1850.
In 1891 Elizabeth H. aged 39 born in Reading appears to have taken up with a carpenter named Edwin Margetts and is living with him in Reading as his wife.
There is no marriage.
So if it was your William who married Elizabeth, it would explain the non-marriage to Annie, and possibly also the move to Wales.