Anacdotal experience only. But I'd suggest it's way more common then folks know.

More so from the 1970s and before. And in rural areas of the UK. But that is more to do with options and how they spread post 1967 law changes. (Pill and abortion both legalized. Though pill only for married women at that time). But for younger readers. Both were often hard to obtain for young adult women in rural areas until the mid 80s.

As a teen My GPs lived in a farming village. And would hint at it when I visited and got to know a few local girls. I did not think much of it at the time.

Post Uni I lived with them for a few months. And got to know one of these girls way better. To the point she felt the need to explain why she never wanted children.

Apparently she knew a few others in the village were in the same situation. For timing this was mid 90s we were both in our mid 20s at the time. So he mother likely finished school in the late 60s. And I know sex education in schools was very hit and miss due to political attitudes at the time. With city schools being more comfortable then rural. Late 60s was when the pill was legalised and labour were pushing all schools to add sex ed classes. So many parents objected.

But news over the last 30+ years. Would lead me to recognise while Insest is no less common. Pro Choice really matters for more reasons then most would expect.