"BH Williams" wrote

You can get the bags from any high street bank and I've never had a
problem with using the "wrong" bank's bag...
Banks and post offices weigh the money (which must be in the prescribed
amount for the bag, £1 for 'copper', £5 for 'silver' and £ 20 for £1/£2
coins), and only count them if there is a discrepancy between the actual
weight and the norm. Bags seem to be in a 'pool', and I've never had
problems with the wrong sort of bag in all the years I've done it.

£10 for 20p or 50p coins. If you are paying coins in, and haven't got a full
bag, they can still weigh what you've got, and only seem to resort to
counting if there aren't very many coins. They do, though, like you to
separate different denominations.

Peter