I am leaning towards the fact that London has a measure of herd immunity.
I do believe London was hit early and hard. The ski season in Europe is big for many Londoners and we also have Chinatown and the two biggest airports in the UK.
I am not convinced that the antibody tests tell the whole story. Do they pick up those partially immune from having been partly infected by other Corona viruses or those who somehow never develop the disease. I know of several people who tested negative despite having very corona like sicknesses early in the year (including loss of smell and taste).
Another theory that I have heard is Londoners are mainly white collar workers, who find it easier to work from home than, say, in the industrial conurbations in the Midlands.
Many around here (London, Zone 3) seem to be very relaxed about social distancing now. We also had the riots. I guess the next few weeks will tell whether we do have a measure of immunity or just not enough contact YET for a second wave.