I have been to St. John twice (U. S. Virgin Islands). The british side seems even more laid back, much less populous and have some great islands and great water -- from what I have read about them. Sounds like a great trip.
The land there can be steep in places and fairly rocky. It might be good to get some hill climbing in your "practice" walks. It seems like flatland walks don't hit the same muscles. And some lightweight boots with good ankle support would be a good idea.
I am very impressed by your workout plan. Have fun!