There are an approximate half a million domestic cats in London. I came to this figure as follows:
There are 7.4 million cats in the UK. The population of London is 8.674 million. The population of the UK is 64.1 million. Therefore the population of London is 13.53% of the UK population.
13.53% of the UK cat population is 1,001,000. That should be the approximate number of cats in London but a lot of people living in London are transient or single and working hard and not in a position to look after a cat and there are a lot of foreigners who might not be so predisposed to look after a cat. Then there is the accommodation. A lot of Londoners live in cramped homes of multiple occupancy where cats would be unwelcome. There is a substantial number of Asians who I’d suggest tentatively do not like cats as much as the typical Brit.
For these reasons I have halved the one million figure to half a million cats in London.
I have never seen a feral cat in London but I am told they exist. If so there are relatively few.
Sources: Google for the stats.