I rode from home (Milton Keynes)-Dover-Land's End-Durnes-John O Groats-home on fixed in 2006.
IIRC I used a 80" or so gear. Except for the bit from Yeovil-Land's End and back to Exeter and the A68 from Edinburgh to Darlington on the way home, where I used about a 70" gear. I did a mostly main road bash along the A30 ( a fair bit through the night) and the old A30 where I could. I was glad that I had a lower gear where I used it! The 80" wasn't a problem (for me, I like bigger gears) going over Shap or over the Scottish Highlands. It's a long drag up from Pitlochry to Dalwhinnie, so expect to be slow. The run down to Inverness from Dalwhinnie will make up for it.