Is there an XC route that's a known quantity?
I know the roadies and the walkers have routes, but is there one that is more XC bike friendly?

there's an article on mtb britain that may be worth a look.

http://www.mtbbritain.co.uk/lands_end_to_john_o_groats_off_road_by_mountain_bike.html

certainly down this end you would need to know where to go offroad as it ain't marked or indeed obvious.

The CTC have an offroad route.
http://www.ctc-maps.org.uk/routes/route/1372

But you need to be a member to download. There's the same route lurking around on the web somewhere, but can't recall where. Googling lejog2 and offroad might work.

i would love to give that ago 1200 miles 10-15 days???

there was a good article in the ctc mag about a young guy who did it on his own and camped.

http://www.rsf.org.uk/routes/uk01.htm
Looks interesting!