I've been on the M6 from Gretna down to Birmingham as a passenger. We usually end up on the M5 heading to family in Gloucestershire. And I'm effectively an annual passenger of the M6 in the Lake District as I have family in Keswick. The borders route of A74(M), M6, A66 is a really nice route to be on.The M6 is nice through the Lake District. The rest of it is absolutely horrendous, so I wouldn't recommed any part south of Preston (except, perhaps, the Thelwall Viaduct).
The parts of the A7 I've been on (as a passenger) are really nice, we took that road one year heading south. I'll also check out the others next time I happen to be in that part of Scotland.I would recommend the A7 highly! I think a lot of the other roads in that area - A701, A702, A68, A70 - are also very enjoyable.
I'm a pretty new driver (since April) so I haven't managed to cover a wide range of roads (driving wise) yet