[wongarsu]

It's not just that there is an increase in cases after the mutation appeared. It's that in the regions where this mutation is prevalent there are many more cases than in other parts of the same country (and in fact case numbers are growing in regions with this variant and falling elsewhere). The other hint is that the new mutation is becoming the prevalent strain in south-east England, which on it's own would already indicate that it has an evolutionary advantage over other strains that allows it to outcompete them. Combined with everything else we know it seems clear that this advantage is faster spread.