I'm 50/50 on it being a bioweapon gone wrong or an initial failed attack followed by Chinese counter-strike.
My guess is, considering their unnaturally firm belief in herd immunity throughout, is their plan was to have a harmless variant spread across their own population first, then deploy a deadly variant in China. The emergence of deadly variant in US and UK could be natural mutation or it could be counterstrike. Halon's razor would say it's natural mutation, but viral mutation into more deadly variants is also unusual.