I believe the overall IQOS sales in the UK are still fairly low (I think around 2-3% market share), but since they compete with a lot of aggressive e-vapor and other products, the fact they are winning is very encouraging.
Success for IQOS in the UK is particularly good for PM, since they have a relatively low market share of the cigarette industry there (estimated at about 7% largely from Marlboro). The UK cigarette market is dominated by Imperial and JTI, which combined have over 80% share.
So any UK IQOS users that PM acquires, will largely come from their competitors and not from their own existing customers. That's very different than a lot of mainland Europe, such as France, Germany, & Italy, where PM has the dominant cigarette market share which gets cannibalized as IQOS grows.