The entire County of London[0] had an average population density of 60 people per acre (38,400 per square mile) in 1911 and 42 per acre in 1961.

60 per acre being averaged over nonresidential land uses meant that it was still common to find residential densities higher than 40,000 people per square mile (15,000 per sq. km) at that time. Only Tower Hamlets and Islington remain around that density to this day.[1]

[0] https://en.wikipedia.org/wiki/County_of_London

[1] https://en.wikipedia.org/wiki/List_of_English_districts_by_p...

Much fewer / slower cars. Nowadays people have been pushed aside to make room for cars.

Most of the popular London markets are very alive these days. You can barely move in them some days. So I think it is the latter.