How far round GB? Good question that. If you're prepared to deviate up to 50 miles from the coast as you travel point to point its around 1800 miles, if you wish to traverse the actual water/land boundary on an imaginary perfectly still day around every pebble on the beach then it's an infinite distance! To answer the question you have to say how much you are willing to smooth out the coastline. There's probably a PhD in it for someone. Great way to spend three years avoiding getting a real job!