It's not unlikely that the new variant is becoming prevalent due to natural selection. Another point is that London, along with other parts of the south-east the main focus of the variant's explosive spread, was, until recently, under very mild restrictions (Tier 2), which are now regarded as having been ineffective. New variant that was maybe SLIGHTLY more transmissible, huge crowded city, indulgent restrictions regime, a penchant for many Londoners to pile down to Cornwall and Devon for their late holidays...As with everything, nothing is simple and nothing can be ascribed to a single factor. My hunch is that the variant IS more transmissible, but that only a fool would put a percentage on it, and we must use that to explain its spread only when we take all those other issues into account.