Yes I agree. Never heard of these, seems totally bizarre, but I suppose I get the appeal (if you didn't know/think about the proprietary connection/protocol) if your kitchen/scullery wasn't designed with space for a dishwasher. But how many properties in the UK can be like that? Not very many, I'm sure an estate agent would tell you you'd have a hard time selling without 'updating'.

I don’t know about the UK, but Paris provide a large number of incredibly tiny apartment without dishwasher and the landlord is not open to make the plumbing accommodation to setup one. If even possible.

I had to pay it myself once in 2013. It was a sunken cost I was ok with given how cutthroat the real estate market is and how much I hate to do dishes.