Hi Brian
It is a known fact that although civil regitration started in 1837, a number of births weren't recorded between 1837 and 1875 when there was a change in the registration law. There are several reasons, the main one being that many people considered a baptism to be sufficient, as it was being a member of a parish that was the way you accessed aid if you became needy. The 'parish' not the state was responsible for you. The other was illegitimacy and the stigma attached to having a child out of wedlock in some social circles.
Thank you for adding that the census has recorded an incorrect age for Maude, the chatters here can only look at the records available to them and don't have the benefit of family information.
As an 'outsider', looking at the birth registrations in the index, it is easy to suppose that the entries you have found aren't the same siblings as Lambeth is south of the river Thames and historically in Surrey and Mile End is North of the river and in the East End of London. The only way to be sure is to buy a copy from the GRO. The actual details aren't online and will require a change in the law and a huge financial investment by the GRO which, as a governemt department, is subject to severe financial constraints.
Researchers don't have direct access to the actual details of births, marriages and deaths as recorded in the registrars registers. We try to use other resources such as transcribed baptism, marriage and burial registers which used to be published by Family History Societies. These are becoming more readily available online usually on subscription sites. It would seem that your family went down the state route and registered the births of their children but didn't have them baptised into the Anglican faith, you tend not to find non-conformist, Catholic, Jewish or Quaker records online. Your family may of course fit into these latter categories.
I mentioned the Banns as couples had to pay to have them called. To keep expenses down, even if they were resident in different parishes, they would have the Banns called at one church only. The other route would be to be married by licence which removed the need for Banns, but this was a more expensive route to go down but meant that the other parishners know that you were planning a wedding.
As Frederick & Amelia had the Banns partially called St Dunstans you would expect to see a marriage very soon after at the church. However in ths instance there isn't a subsequent marriage there or at any other Anglican parish in the area or in the indexes for 1865. If they had married at another denomination or the register office you would expect to see an entry in the index. If you need to pursue this, you would need to contact the local register office and ask them to search all of the other marriage registers for that registration district. I know that the GRO do not have the facility to search if you think an entry is missing but you don't know where specifically the marriage took place.
Hope this helps.
Dawn