Yep. You are one of my reference points for "EE then Three" as a recommendation for canal mobile internet. We had very few problems with Three when we were gallivanting your way last year, and all of them could be solved by moving a bit further so we got a better signal.
They might also have been fixed by stopping earlier, but that's not what we did.
For clarity, we are Three only via mobile phone in the window. I don't get enough spots where I can't get signal to bother with a good router and a good external aerial, and we need to be able to process card payments for sales off the boat ...
Maybe your favourite pub moorings don't like the Three network!