I also suspect that the birthrate is already slowing down in 1911.
Britain was a huge baby factory during the latter 1800s - as we can see in the census. The country was spectacularly succesful at producing huge numbers of people to populate an empire
maybe 3-7 is the most common number of kids rather then the 7-14 of 50 years previous
The number of births registered 1881-1890 was 1,778,048, for 1891-1900 it was 1,831,030, and for the years 1901-1910 it was 1,859,642
The number registered in 1903 at 948,271 was the highest from the start of civil registration.
Office of National Statistics
Stan