I guess it's too late but if I were you I'd start in John O'Groats and head South. When you get to the end you'll be in the middle of nowhere, it'll probably be really windy and raining and you'll have to turn round and cycle another 50-100 miles to get to a train station or bus depot or whatever to head South again! Ending up in Lands End leads to something like a 10 mile ride back to a main train line which would get you all the way to Southampton, Pompy, London or wherever you want to end up. Just a thought .
Edit: There's also the best fish and chips shop in Redruth that you can stop at on the eve of your last day before rolling the final 25 miles to Lands End. By the way, are you rough camping on the way up/down or what?