I just got back from a week driving around England and Scotland and I noticed that most short motorway-grade spurs from mainline motorways are included in the system, but not the M4 spur at J7 to Slough.
Highways England GIS classifies this similarly to the Heathrow Terminal 5 spur and J4 Orpington spur from the M25, which
are in TM, while surface signs show the 2/3-mile trumpet stem as part of the M4.
Since there is precedent for including short motorway spurs like this in the UK in at least two places, I propose we add a new
M4SprSlo from M4 J7 to the A4 roundabout.