I'm pretty new to the world of single speed, so please take anything I suggest with a pinch or two of salt...
If you know the route you plan to take, can you get gradients for the various climbs? I'm pretty sure that various bike GPS devices allow this (I know my Garmin does), not to mention on-line route planners and even OS maps?
Once you have these, check them against your local climbs that you find easy, hard and impossible to do in the gear ratio of your choice.
After that, it's just a decision based on how many of the C2C climbs you think will beat you and if you are willing to walk those. Of course, if push comes to shove (or walk) you could always drop your gear ratio - sacrifice a bit of speed on the flats and down hills to gain a bit on the climbs?
Just my 2p worth
J