So all cyclists should use a cycle track/circuit that is only a couple kms long and completely flat? (This is not to say cyclists don't use it, it has been very well used as a speed circuit but therein lies the problem) You can't use the promenade if you are doing proper cycle training so that's out.
OK people say, stick to the B roads. Well most of those don't have a decent shoulder plus you have to keep stopping at lights or stop streets which hinders a decent cycling workout.
By in large, the M4 is the only decent road to use if you need to do any sort of distance training, its long, in most places has a decent emergency lane to utilise, and has varied elevations.