I think common sense applies here - it is not a mandatory caution, but there is enough framework within what's written in the laws of the game to issue a caution if required, likewise there is also enough for you not to issue one. As the referee, you have to judge on the merits of the incident and in the context of the game, whether or not a caution is required.
The laws of the game (not FA Caution codes) allow you to caution in instances of 'unsporting behaviour', which are further defined below it. You could use 'commits in a reckless manner a direct free kick offence', 'commits any other offence which interferes with or stops a promising attack' or 'shows lack of respect for the game' as possible reasons to issue a caution here under USB.
The only thing that is clear, is that you cannot caution in this instance for delaying the restart. It has to be USB.
The laws of the game (not FA Caution codes) allow you to caution in instances of 'unsporting behaviour', which are further defined below it. You could use 'commits in a reckless manner a direct free kick offence', 'commits any other offence which interferes with or stops a promising attack' or 'shows lack of respect for the game' as possible reasons to issue a caution here under USB.
The only thing that is clear, is that you cannot caution in this instance for delaying the restart. It has to be USB.