There’s no extra gate or dedicated staff member in my store only whoever’s at the till and if the self-checkout is busy they’re too busy to watch them.
The difference is other countries have much larger stores… probably because we have a more car centric culture.
My local store has about 40 checkouts - half of them self checkout. And there’s a competing store literally door (in the same building, with ain internal wall separating them), which sells all the same stuff and is the same size. In the middle of the day about half the checkouts are open and in the evenings all of them are open.
We do have smaller stores like yours, but almost nobody shops at those and even at peak hour a single checkout is enough.
Sales so slow at my local small store the checkout staff will literally check your bread for mould when they scan the barcode… They’re more expensive and the food is worse.