Mandatory speed limits are indicated by a figure surrounded by a red circle, which applies to both matrix signs and permanent physical signs. The exact wording of the Department for Transport guidance is as follows: "Where variable speed limit signs are mounted over individual lanes or at the side of the motorway and the speed limit is shown in a red circle, the limit is mandatory. Speed limits that are not shown in a red circle are the maximum speeds advised for the prevailing conditions." You can read more about this in detail on the government website here - www.gov.uk/government/publications/know-your-traff...s

Even if a speed limit is advisory, excessive speed beyond this limit that is deemed inappropriate for the conditions could still result in a charge of driving without due care and attention or dangerous driving if witnessed by the police.