I was born in and have lived in the UK for my entire life.
Although this document correctly asserts that we use imperial units in certain use-cases, my anecdotal experience is that I don’t really understand those units at all.
When I buy milk, I know roughly how much “2 pints” is, and I know roughly how long it will last me.
Similarly, when I see a distance in miles I mentally multiply it by 1.6 to roughly estimate what the distance is in kilometres.
When people ask my height, I often give it in both feet/inches and centimetres, but the former means nothing to me. It’s just some words I’ve memorised that directly translate to my height.
My point is - I see a divide in the UK. I was born after 1990 and I think many others like me have a similar relationship with imperial units - they don’t “think” in them or understand them at all.
It’s my parent’s generation who seem to understand and think in imperial units.
I grew up in a metric system country, and moved to the US. Miles are not a problem, I'm driving 60mph, I see that I have to drive 120 miles it will take me 2 hours. A pound is roughly a half kilo. A pint is about half a liter. But what drives me mad is Fahrenheits after 15 years in the US I still struggle with it.
It sounds like you have a similar process to me - you don’t actually “think” in or understand the imperial units - you have just memorised benchmark amounts and conversion metrics. That’s why you can’t get your head around fahrenheit - the conversion isn’t an easy one to mentally apply every time.
I'm in my mid 50s in the UK and I was only ever taught metric units - I have no idea how imperial units work. Miles are a unit I only use when driving (and always think of it as 1.6km) and distances and heights seem natural to me in km and m (even though I am working my way round the Munros!).
