Traditionally, for runs South of Edinburgh, SE krews meet at the Esso just where it says Hillend on your map.
Then A703/A701, dive off to the left just before the short dual carriageway leading into Penicuik, which is the B7026 via Howgate - it's a 40 most of the way these days, but still quicker than Penicuik.
At Leadburn to do the motorways you would go straight across and follow A701 all the way to the M74/M6/A14/M11/North Weald.
To do the A1 you would follow the A703 (left turn), A72 at Peebles (left turn) as far as the fork with the A707 which you follow towards Selkirk, then cut across to the A7 through Selkirk (there is a natty way to do this, but cannot quite remember it, so just follow the main road will be fine). A7 to Hawick. At the round-abouts in Hawick town itself you take the A698 which is straight across the first one and left at the second one cutting back to the North.
A couple of minutes up the A698 (just as it turns to national speed limit) turn right onto the A6088 Boncester Bridge road (it's awesome, but treat the early part with respect as it has some severe bends). Once you climb the hill (and it is a bit like a hill climb, though not so narrow) on the way out of Boncester Bridge the road really starts to open up and some silly speeds are possible.
Then merge with the A68 just in time to say goodbye to Scotland, hello England, goodbye speed cameras

A blat down as far as Rochester (old roman stuff here if I remember) and then stay straight at the junction with the A696, following the A696 all the way to the A1 at Newcastle. The last 5 miles are a bit built up, but doesn't take too long.
An alternate route, which takes in the A68 roller coaster is to turn right here and follow the A68 to just before the A69, then run alongside the A69 on the B6299 as far as Willington, then A69 to Newcastle and the A1.
Either way, from here for Walshy's I would suggest A1/A1M all the way to Peterborough (about 200 miles down the road), then A14 east, M11 south to J7 and then the rest of the way on local roads.
As I will almost certainly be coming along behind you, and we have two rooms booked, you guys can choose to share and I'll pay the whack for the other room, or you can take a room each and leave a note at the front desk for whichever room I should invade on arrival - as usual, I am happy with whatever you want to do.
Cheers,
Robin