My understanding is that it was never required to sell things in metric, just to also include metric values. So you could still but a pound of bananas, but it had to also be labeled as 454g (or whatever a pound is in normal units).

And yet my English uncles are constantly telling me about how many stones they weigh, and my English aunts sending recipes that say to turn my oven to "gas mark 4".

Or masturbate in the daytime

(a pint is closer to half a litre but I don't want to be that pedantic)

In South Australia (it varies by state and it's a whole shamozzle) we call them pints even though they aren't "pint" sized. It's just a word for "the big one". And if they sell imperial pints, they will suggest it.

But you can just say "go for a drink"

The Brits were the largest economy in the world for the 19th century, and the largest in Europe until WW2, and followed up by the US. Being the largest economy means you get to keep your own internal standards and anyone who wants to do trade with you has to accept that. It's not a coincidence that the UK swapped to metric after integrating into the European market.

pre-metric systems were perfectly sensible in the time they were made for, it's when we started standardizing things and building precise machinery that they stopped making sense.

In the past there just wasn't as much of a need for it, you don't need standardized units and measurements to simply make something that functions, and when sharing measurements with others you can either get by with a vague "about 3 foot 4 inches" or you can literally give them the measurement on a physical object.
And when working like that it's actually nice to have these kinds of units, an adult thumb is roughly the same no matter who and most people have at least one of them on hand.

And by "making their own" they put a wig and a pair of sunglasses on the British Imperial system.