You should always drive within your limits of the road and your vision. The national speed limit of 60mph is not an order that you MUST drive at that speed - it's simply the maximum speed permitted on that section of road, but ONLY when safe to do so. If it's foggy or the traffic is heavy it's certainly not safe to hammer along at 60mph! Similarly, if you shouldn't drive at 60mph if you can't see enough of the road ahead to stop or slow down safely. Those like Richard who "came round a bend and had to brake sharply...." are driving far too fast for the road conditions.
Suppose there was an accident just around the bend or a tractor, a bus stop or other traffic stopped just out of sight? It would be reckless to be driving so fast you couldn't avoid it - regardless of the road having a 60mph maximum limit. It's the same when there's a change of speed limit just around the bend. It's up to the driver to drive safely if they can't stop or slow down safely within their line of sight.