SiN is only one effect of separating cyclists from the danger of motor vehicles. If there wasn't an extensive Dutch cycle track network nowhere near the per capita mileages would be ridden. It's disingenuous to suggest numbers alone account for the safety record divorced from the trackways on which cycling happens.Yes, the Dutch benefit from the Safety in Numbers effect which means their death rate is lower than ours by the amount the SiN effect would predict.
It's reasonable to argue topographic differences between the UK and Netherlands for implementing cycle tracks but their positive effect on number cycling and mileages ridden when the Dutch model is applied is self-evident.