MIG wrote:

Staff were told that we could still accept ½p coins from customers,
but only in pairs. This was strongly emphasised and always struck me
as bizarre.

It sounds like a customer friendly move - "We still accept your out-of-date
coins" - as well as an way of ensuring people suddenly have, for want of a
better term, credit that can only be used there.

Presumably Sainsburys had an arrangement whereby it could cash in all
its ½p coins by some deadline, but even if staff accepted them not in
pairs, the entire Sainsburys chain could only ever have been stuck
with one odd ½p if they ended up with an odd number overall.

Well when would anyone have reason to pay a sum ending in ½p? And how could
the store convert or give that back in change?

I also wonder what happened to anyone's bank balance that ended in ½p.