Hm. UKMPG are persistent little buggers, aren't they; from back in 2/2019:
https://uk-ports.org/ukmpg-cost-bottlenecks/. (Compare with
http://www.doverport.co.uk/about/news/open-letter-from-doug-bannister,-ceo,-port-of-dove/13553/.)
Interesting that the perceived threat seems to be delays in manifesting or EU export procedures at busy EU ports (or perhaps incoming Transit ones at busy UK ports), rather than the whole 'gridlocked Kent' thingummy. Like the last time this came up, whilst it's prima facie cool to avoid putting all your eggs in one basket, some of these 'alternative' ferry routes are rather on the long side, meaning that the daily uplift capacity of a ferry working them is somewhat constrained and the benefit, how shall we say, somewhat marginal. Still, if it keeps Hull in ferries I guess they for one won't be complaining...
https://www.bbc.co.uk/news/uk-england-humber-54377287.