*BOOK WELL IN ADVANCE!*
This is particularly true if you embark on a journey of this sort during school holidays. All train companies have limited space so booking yourself and your bike on a train is a must. Do this in person and not over the internet. While the internet is a fantastic tool for a lot of things, I couldn't find a train company who would take reservations for bikes on their sites. However, I waited a bit and got a really cheap fare to Penzance, having phoned them and being put on hold for 28 minutes!
You can, of course, always get a friend or relative to give you lift to the start or from the end, or even take a one way hire vehicle. For me, though, taking the train was the easiest.
With a company like bikeADVENTURES organising the whole trip for you, they also take the worry of getting your bike 'there & back'. For a fee, they will pick your bike up and courier it to Land's End and do the same for the way back. This is particularly useful if you plan to travel by air, as baggage handlers are not the most understanding of cyclist's needs!
Included in my holiday was a coach trip from John O'Groats to Inverness making drops at the train station and airport. I decided to travel back by plane as this was the easiest and, as it happens, the cheapest! I will say 'goodbye' to my bike, as it will be couriered and I will catch a plane from Inverness to Gatwick, then a quick train journey to where I live.