M6 is a huge tyre squarer. I did it once from Northants and ended up with square tyres.
Not being funny, but if you want to keep the bike safe, avoid cities. Plent of decent camp sites or B&B's available.
If you vcan, switch across to the A1 and go up through Jedburgh from the top end of Newcastle. SUPERP roads through the Borders and well worth egtting a B&B there.
I did a day on track and then rode Corby to Stobo near Peebles easily enough in a day. My best time was just under 4 hours averaging 84mph inc sticking to the fixed 50mph and no silly speeds and obeying any under 60mph speed limits, but I had done that run a few times to see my ex. Using the car, my best time was 5 hours and my worst was 12. but on average it was around 6.5 hours.
Oban to Fort William and all round that area is fabulous. Put out a post for some local 400cc guides!!!! Did that last time to get even better slightly off the main route roads.
M6 is Dull too

I agree that the Oban to Ft William road is superb, I live in barcaldine on the way from Oban to Ft William. Lovely roads, hardly any coppers!! Last year my girlfriend and I did Oban to Skye then to Inverness then Aviemore then Crieff and then back to Oban again, overall we took our time and did it in 5 days. It was well worth it and would deff recommend that route

I have done South London to Holyhead (the northwest tip of wales). that route was north to birmingham and then cut across wales along the A5. Google maps tells me it will take 6.5 hours, 310 miles. the last time i did it in just shy of 5.5 hours including generous breaks and a photo break.

After the 310 miles in the UK i did another 160 in ireland so its definitely doable.

im thinking of going up to scotland around july, will probs be going up to broxburn for a couple of days to see family / visit edinburgh with the missus then shoot up to aberdeen to visit my uncle :) from there it'd be a long ride round the highlands maybee visit skye then back to broxburn via glasgow.. on the way back i'll check out hadriens wall and spend a night in the lake district. all just basic plans right now but im thinking of it :)

even then would it be wise to take the missus on such a long road trip on the CBR lol...