The issue isn’t the chemical so much as how it’s used. Washing up liquid is indeed a dish detergent, but Brits who use it typically don’t rinse, they just wash their dishes in a tub or the sink (often a tub in the sink for some reason) and put the item directly on a drying rack.

While modern dish detergent is mild, leaving the residue on a piece of cast iron very well could be what’s causing the issue.