I did not go through all of it, but there was one feature that I had to mention, because through the parts that I did complete, it was a consistent issue.
While I understand the need to have the submit button on a given page as the learner can try multiple times to get the information done correctly, once the learner has answered correctly, the submit button should disappear. It was not always intuitive to me to click below to the next button. My natural inclination was to click again on the submit button. Perhaps when the learner has responded correctly to an exercise, the submit button can change to a "continue" button, which is actually just another next button really, just so the learner can move along. That feature just stuck out at me a lot. Does that make sense?
Otherwise, I would agree with commentary above, and I'm sure you are still working on it. I love the topic--it made me want to go back and visit the UK again!
I hope this helps, amiga.