Indeed, the rules referenced here state:

Single waved: Drivers should reduce their speed and be prepared to change direction. It must be clear that a driver has reduced speed and, in order for this to be clear, a driver would be expected to have braked earlier and/or discernibly reduced speed in the relevant marshalling sector. Drivers should not overtake any car in a single waved yellow marshalling sector unless it is clear that a car is slowing with a completely obvious problem, e.g. obvious accident damage or a deflated tyre.