Well, having now completed my ride I can answer this question. Firstly, it was a great ride, well planned if I say it myself, and everything worked 'perfectly', but not so perfectly that I couldn't think of ways to improve it.
So things I'd do differently:
* I wouldn't use the same route. I'd still be happy using the Royston Woods route as a basis, but I'd keep to the roads as far as possible and, in England, only use cyclepaths and canal towpaths where there was no reasonably quiet, reasonably direct 'safe' road alternative. In Scotland I wouldn't use the route on the A9 beyond Tain, I'd try heading up via Lairg.
* I wouldn't take my Gopro and accessories. Never used it, added weight, and my phone was adequate for my video skills and inclinations.
* I would take some chain lube. My friend did so I was able to use his. It helped keep the drive train running sweetly applying it a couple of times during the trip, although I had cleaned and lubed before the start.
* I wouldn't take 3 spare tubes, just one. Ideally I'd fit some tubeless tyres. Neither of us had any punctures, despite some of the surfaces we rode on.
* I can't really think of anything else!
Other than that it went very well, the daily distances well suited to our abilities and style of trip (unsupported B&B). I still don't know what sort of cycle tourist I want to be, or even if I want to be one - I think my natural 'style' is more 'sportive' than 'touring' and I can't say I really enjoyed the extra weight and drag of panniers, but at the same time I did like doing something other than a circular route, using the bike to travel rather than exercise. I don't think I'll do LEJOG again, just because there's the rest of the world to explore, but I'm really, really happy to have done it.