Am I right in understanding that the fuse is there to protect the cable against fire in the case of equipment faults, since UK rings have a larger current capability (depending on the circuit rating) than say EU star systems that may have lower ratings. If the cable is capable of high current as in most specialised cables, then it does not matter that the fuse is 13A. If however you have a dedicated lead for equipment that only generates a few watts, for example my TT PSU then the fuse should match the cable so in this case 3A.