The only route I can think of for this is:
Wick (you may be talking 0400/0500 in order to get a path)-Inverness-Elgin-Aberdeen-Dundee-Perth-Cumbernauld-Wishaw-Carstairs-Carlisle (then in all honesty I would have to say Cumbrian Coast or S&C, I doubt you could do it via Shap)-Preston-Warrington-Wolves. Then the fun starts. Can you get a path through Birmingham. It would deffo have to go via Camp Hill, maybe through Walsall and Sutton Park-Temple Meads-Exeter-Plymouth-Penzance, though long strectches of SLW in the Highlands and pathing in the North West/West Midlands could snnoker you.
Mainly to avoid pathing issues, and to make for a more scenic route, I was thinking the following (lifted directly from Page 1, barring Wick):
Wick - Inverness - Aviemore - Perth - Stirling - Cumbernauld - Carlisle - Settle - Hellifield - Preston - Warrington BQ - Chester - Shrewsbury - Hereford - Newport - Bristol TM - Taunton - Exeter SD - Plymouth - Penzance
It also means that many of you will be able to experience a Pacer in a lengthy tunnel for the first time!
I would suggest doing this for Children in Need, but for the route to work best it would need to be undertaken during the summer. This is because paths are limited north of Perth (both ways), and unless we go at night we will struggle. But what is the point of going at night if it is pitch black? Thus, by going in June, it would be much clearer, even if we left at 04:00. Thus, unless (very) delayed coverage would work, this idea would fall flat.
I'm sure paths could be found for this. This route avoids the sections that are likely to cause problems (Shap and Birmingham especially).
Or, alternatively, we could revise the route between Preston and Chester, and divert via Ormskirk. It would require the removal of the buffers [assuming there are some there], but otherwise this section could be the overnight part, and there could be a media stop at Liverpool Central (by which I mean the media hop on board and talk to us for a while; it would also make an ideal rest stop if Merseytravel agree); we could then use the Halton Curve and resume the above route at Chester. This idea would avoid the WCML through Wigan and Warrington, whilst also making for a good opportunity for a "pit stop".