It looks like everyone's having a go at constructing plans for the junction, so here's my attempt (mapping data ©
OpenStreetMap contributors):
This is an "inside-out octopus" design with a roundabout at each corner, allowing the junction to be fit into a smaller space via the use of tighter turns, and allowing access to the local roads (something that a regular octopus can't do). The geometry isn't "final", rather it's a product of my lack of art skills; the southwest roundabout should probably be moved further from the junction to give more room for the loops. The "collector road" running parallel to the M1 to the left of the diagram is just a slip road that merges into the M1 a little off-picture. I'm assuming that there will be a fairly low speed limit on the turning movements, but probably not signals; there are no conflicts between any of the 12 "main movements" between the expressway and the M1, so the junction should flow quite freely despite not being freeflow (and it could be made freeflow simply via adding cones, although that would cut off some local movements and lead to a junction with excessively tight corners and the occasional hairpin turn; the roundabouts help mask those issues, and in fact some real-life junctions contain apparently unnecessary roundabouts for what I presume is this sort of reason).
One way to think of the design is as a large anticlockwise stackabout with a smaller roundabout in each corner for local movements; you join on the inside and leave on the outside (thus meaning the major right turns have no conflicts), and it's possible to loop round the whole junction anticlockwise until you reach an exit for the road you want, thus it's full access (although some local accesses, like A421-from-Milton-Keynes eastbound to A421-expressway northbound, require navigating pretty much the whole junction, they're still at least
possible). Dedicated left-turn sliproads are added to ensure that the left turns have no conflicts either. (Unusually, they're on the inside of the junction; placing them on the outside would conflict with the local roads, and there's an obvious place to put them which works out pretty neatly.) If it turns out to be necessary to give some of the local routes shorter paths, then some of the roads connecting the roundabout could be made two-way, thus converting the design into a "magic stackabout" (and one which
still has no conflicts between the twelve major movements!).
