How can this meet the regulations for social distancing etc?

As well as the suggestions here, you could ask people to minimise touching items unless they’re fairly certain they will buy them.

I think you are being over-cautious. Groceries are touched by various people - factory conveyor belt worker, the packers, delivery drivers, shop assistants putting the stuff on shelves, then customers, check-out staff. Do you think all of those people sanitise their hands/the items? No.

The stallholders should be washing their hands, then the customer can wear a mask (if they feel better by doing so), then they can wash their hands when they get home.

Just a thought, but if this is going to be outside, surely the stall holders will not have to wear a mask?

Ask everyone to use hand sanitiser as they enter the churchyard, and have a dispenser of the same on each stall. If people’s hands are clean when they pick up stuff, there’s very little chance of them passing on the virus.

As well as the people in he stock room, the shelf filler and the factory/farm.

Take the precaution of not touching your face until after, then put any items to one side and leave for 72h, then wash hands thoroughly for 20sec - surely that's enough?

Well Yes you are being over cautious because everything is being touched in every shop, every day, if you go to a supermarket and pick up a tin of beans then decide you don’t want beans, you put it back on the shelf
I m working in a charity shop so I m always tidying up and putting things back on the shelf, they ve been touched Then the check out shop assistant may touch the goods you hand her to check the price or the label everything is being touched all the time

The stall holders could wear masks and stand back behind the stalls?
Change still has to be given ( but it does in shops) so the helpers could use hand sanitiser every so often.
It isn’t an ideal time to do this, but if it really has to be done there are ways around it by being careful.

I dont know specifically about this but in the shops people pick things up and put them down - did it myself in Sainsbury this morning. No-one is sanitising the items in there.