Look at the prevalence graph in the article, you can see that the new variant has been gradually taking over the turf from the other ones. This cannot be explained by just the founder effect unlike most other cases, since the prevalence was high to start with. It cannot be explained away by a single super-spreader event, since a single event will just cause a single step-change. This has been a continuous process.

It could be random chance or a selective advantage, but then it comes down to just a modeling exercise. How likely is it that this could happen by chance? And it appears quite unlikely: instead the best way to explain the data is a significantly increased transmission.

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.

This variant was not dominant in London some weeks ago, and now is.

> Could it still be that this variant is as infectious as others, but just happens to be the dominant one in the regions where the virus circulates the most, for reasons that have nothing to do with the genetics of the virus?

Yes, but one assumes that confounding factors like that are fairly easy to isolate. At some point, especially as the growth continues, statistics pretty much rules out other explanations. General consensus is that this is almost certainly more infectious. But sure, there's always more science to do.

> Could it still be that this variant is as infectious as others, but just happens to be the dominant one in the regions where the virus circulates the most, for reasons that have nothing to do with the genetics of the virus?

That can happen when general incidence is very low, in a small region, or for a short time. But this is not any more the case here, and having this happening by chance is very unlikely.