Unfortunately the major issue here is the use of the term British Isles. In reality the trip Spall made was around Great Britain. To travel "round the British Isles" would almost certainly have meant travelling to Eire, as one of the three largest land masses of the British Isles. To circumnavigate the British Isles would have meant going around Eire through the North Atlantic - especially as he did not circumnavigate Scotland, instead adopting to only encompass around half its area missing out Highland, Eigg & Muck & Rum, Skye, Lewis, Orkney and Shetland.