German living in UK here. I think the flaky-currency argument is non-sense. I see this argument mostly in anglo-american articles when the authors try to figure out why germans are still mostly cash-transactional.

I paid in cash back home mostly because it's faster. Cashiers are used to calculate the change and quick getting it out. CC/Debit card payments take a bit longer, since you almost only have chipped payments, get the device to work, enter pin, wait for receipt that you have to sign sometimes, etc. I was never pick pocketed on the street, so this wasn't an issue either.

I only started paying by card with the introduction of NFC/ "contactless" payments, since this one is faster.

Also, the UK coins are quite heavy having €10 worth of coins is half the volume at least compared to the old pounds.