The A41 between Aylesbury and the M25 (at Kings Langley) is a fast dual carriageway. But, it isnt boring. There are some nice sweeping curves with excellent visibility. Traffic is rarely heavy and the trip from start to finish is probably 15-20 minutes if you keep to the speed limit.
One of the best bits of the A40 - some nice fast sections with sweeping turns plus some tasty tighter bits around Aston hill. Need to slow down in the villages en route! Also you can visit the Hellfire caves at the start and Hughenden Triumph at the end!
Latimer road is a nice run with some (short but ) fast straight sections and some nice bends. Only real problem is traffic - theres usually quite a lot! Beware the speed cameras as you enter Chesham at the end of the run.