The Loyals where standard fleet tenders operated by the RNXS (Royal Navy Auxiliary Service), in times of tension they would be used as harbour inspection vessels, ie interrogate in coming vessels examine them then give them an anchorage or berth. They where not intended for towing by this department. Though all F/T's where fitted with a small tow hook on the aft end of the accommodation, as the RMAS would use them to tow small objects very occasionally. As to the yellow stern light the Sal class vessels frequently used to tow bomb targets etc around the coast and they where never fitted with them, the yellow light may be fitted now as standard, but when first introduced, I do believe the M Notice I read said it was optional, expect now it is compulsory for new tonnage.
After the Marchioness incident, there was a new requirement for small passenger boats to have a certain amount of reserve buoyancy in event of a collision. The RN F/t's did not comply with this figure, so the MoD sold them all very quickly, although it turned out the requirement is for new tonnage not old tonnage. If it did I doubt wether some of the present old preserved tonnage could still operate.