Self-checkout works well for half a dozen items in a hand basket. If you have a trolley full of a week's shopping, you're going to use a cashier. In fact, in this supermarket, you can't even get a trolley into the self-checkout area.

My local M&S store has three different checkout areas. (1) traditional, for people with trolleys. (2) Self-checkout area for people with baskets. (3) Really fast (but very narrow) lane for people just holding a few items in their hands. You almost never see people using the "wrong" area.

What's interesting is that I usually pick the one with the longest queue, ie line 3. The cashiers are really fast so you don't have to wait long.

One other factor is that a lot of us in line 3 are using contactless (Wave & Pay) cards, so the payment process doesn't slow things down. People who don't usually use the waiting time to get their cash ready.