In the Midlands, it's more analogous to I-5 in the Central Valley. The 6- (being upgraded to

lane M1 serves the population centres while the A1 doesn't. Parts of that bit of A1 are old and not ideal, but the 4-lanes between Peterborough and the M18 are perfectly adequate now they've finished removing roundabouts. It also doesn't help that when they widened Alconbury-Peterborough to 8-lanes due to traffic, it's been consistently considered overkill. Certainly it's empty compared to other parts, but it would be very busy if 6-lanes (as they will see when the A14 works move where the traffic merges further south, but only 6-laning the difference). And if the A1(M) through Beds was built as they assumed it would when building the Hunts bit, then it would be less OTT looking.
North of the M62 it is 6-lane motorway to the NE (almost finished), but in the NE the motorway is old, and the region neglected (plus the more pressing issue of the Newcastle area, where it's an overloaded 4-lane non-motorway freeway that they have done some widening of) so it isn't 6-lane. Near the border, there's very little traffic, so the coastal S2 is only annoying if you get stuck behind a truck: it can easily cope with the traffic. And then it gets busy again in Lothian, and is 4-lane freeway to suit that.
It's worth pointing out that the old song "Take the High Road" is about the uselessness of the A1 as a fast road to Scotland - "I'll take the High Road and you take the Low Road (A1) and I'll be in Scotland before ye." I'm not sure whether the High Road is the via Scotch Corner (presumably the choice, given the name) and Carlisle route (A66-A6-A7 / A66-A6-A74), or one of the routes via Carter Bar (A696-A68 from Newcastle, or A68 from Darlington). Either way, Berwick is out of the way and so the A1 north of Newcastle is sort of a dead end.