[spuz]

I don't know enough about epidemiology but I would imagine they have done modelling to determine the likely transmissibility or R number of this new variant to be 70% higher. The probability that this new variant has come to dominate by pure chance must be small.

You are right that London is a densely populated area prone to easy spread for the virus but the same must be true for all variants. This variant started its existence as a single strand of viral DNA and has managed to spread far enough to become the dominant strand against competition from many other well established variants.