Some folks I knew of and good anglers who would consistently have good results would place orders from a guy who dug (not pumped) for large blacks and keep in aquariums for around a month till next digging tides.
This was around 2012 and small group would buy all he could supply and split, then for quality dug worm they were paying 70 pence per worm and ended up happily paying £1 or a little more, to them bait was king.
Was tempted myself as not labour intensive but stuck with fridges, cat litter trays and tending daily, when the source of dug blacks dried up they all sold their kit.
If could wave magic wand the worm fairy would hand me a bucket containing a little water and a couple of dozen of very large black lug every time I fished for the cad fash.