We recently returned from our trip, thanks everyone for your suggestions, they really helped!
Hey Madcats, a huge thanks for the suggestion about getting a National Trust family membership! Our first trip to a NT property was Corfe castle in Dorset, while staying at Center parcs Longleat, and we opted to spring for the membership (hubby was impressed with the welcome ctr, parking and clean loos prior to the castle ticket office!). We ended up stopping at 1 or 2 NT houses/parks during each long car journey, which was very helpful. It was very helpful during our 2 week stay in Lake District, as many of the places we wanted to visit anyways happened to be part of NT, and we discovered a few new places too.
We ended up going to Blackpool from the Lake District! Took the kids to Blackpool Zoo, then Blackpool beach for fish n chips, Mr Whippy ice cream and hot mini donuts. Hubby and kids had never experienced a British seaside so I was determined we do it all (minus the rock candy) ..I wanted to take the kids on donkey rides along the shore, but we hadn't brought spare shoes and cringed at the idea of wet sand in the car on the return journey.
The weather was completely bonkers. It went from cold, overcast (sometimes rainy) and windy each day to bright and sunny at 6pm, when most shops were closed and we had just dragged the kids back home out of exasperation with the cold and wet , and were doing mundane things like cooking dinner, doing laundry, giving the kids dinner and baths. While in Bath, we managed to buy rain boots for the 2 older kids, and really put those to good use when faced with several days of wet weather later on. There were a few really bright and clear days during our trip, and we mostly got to do fun things then. The whole canal boats idea got me thinking about canoeing, so in Bath I tried canoeing with the 2 older kids, and it was very memorable, esp as I had no clue about boating/rowing apart from watching "Swallows and Amazons". We then went canoeing as a family in Center parcs Longleat and it was a lot of fun.