Can help here a bit:
Shakespeare - excellent marathon, one of my favourites. Although in April, always hot! There is a legendary 5-mile stretch of cindery cycle track in the latter stages called The Greenway which has been known to turn into a furnace, a crucible of suffering. Fills up very quickly, so get entering asap. Traffic is bad getting to it and parking very expensive - suggest use P&R.
Abingdon - south of Oxford, mid-October. A PB course which always has a fast field - everybody chasing PBs, I suppose. A club runner's marathon. Extremely well-organised, starts and finishes on a running track. Fills up very quickly.
Gloucester - after a dull few miles of housing estates, 3 laps of an attractive rural loop (or 4 if you're doing the 50K!!), two minor hills each lap. PB course.
All three of these I would rate as fast courses.