Just looking at this junction bothered me, because it could so easily be rebuilt so that the M25 is the through route by building a new mainline along the line of the slip that currently carries the anti-clockwise M25. Otherwise, get rid of the now-redundant clockwise M25 slip and keep the loop from ACW M25 to the A21; CW M25 to A21 is just a simple left-side slip. For access to the M26 from ACW M25, a tunnel could be dug under (or a bridge over) the new M25 mainline. The only tricky bit is getting from the A21 to M25 northwards - now effectively a right turn - and to the M26. A left-turn slip from the M26 to A21, as you say, would be very easy but for those pesky houses in the way. Just a thought, but it might just work.