Hmmm ...
can't comment on the English part of the route at all ...
If I were to to a LeJoG, I'd probably go through Edinburgh and sleep in my own bed for the night.

I also think that there are more quiet options on the eastern side of Scotland, up to Inverness, with some lovely wee roads (admittedly hilly, so not ideal for heavily loaded bikes and limited time)
There is also the east coast of England, I heard the "Coasts and Castles" part of the NCN 1 is good (and not too hilly), but that may be too far out of your way if you are wanting to go through Glasgow.
Comments on a westerly route:
Somebody already mentioned that the "old road" parallel to the A74(M) is now quite quiet, I've heard that elsewhere.
I would definitely try and avoid the A82 north from Glasgow as fas as possible. If you are determined to go through Glencoe, another option could be to take the ferry to Dunoon and head along the A815 to Strachur, then Inveraray, DAlmally and along the B8074 to Dalmally, where you can join the A82 again. (less direct, but also less busy and you'd get to see some areas off the beaten track).
[Mind you, if I was doing this and heading up along the western side of Scotland, I'd either go via Arran and Kintyre, and then
a) either hop across to Portavadie from Tarbert follow the B8000 north (serious chevrons!) and then via Strachur as describe above or

weave my way further north along the Kintyre peninsula past Kilberry (lovely Inn and great views across to Jura and Islay!) and then past Kilmartin (zillions of standing stones & stone circles) and along Loch Awe to join the A82 [via the B8074 as mentioned above if you want go through Glencoe] or carry on along the west coast to Balachulish (where I'd divert via the Corran Ferry to Ardgour and Moidart to Lochailort and then on the old road to Mallaig, over the sea to Skye

, and then back to the mainland via the Skye Bridge, and then up the west coast]
Hmm getting lost in too many tempting options here

, here are some more comments on your route:
I'd most definitely go along the SE side of Loch Ness rather than the SW side (much better views and a lot less traffic!)
North of Inverness I'd avoid the A9 and head further west - Strathnaver to Bettyhill as several others have said already ...
Good luck with your planning and the trip!
T