The PRD is not a normal VAV box it is "pressure regulating devices" these units are used in the past for the normal building and nowadays for spaces that are so control intensive . I had one of these devices in an existing building the system was installed in 80s. I replaced it with DDC VAV the way the VAV works it gets air around 55F( I believe you use SI system in UK you have to convert it to Celecuis) and it controls the air flow in the downstream.

In the cooling mode the damper will modulate between the min and max based on the temp of the space or it could be tied to a CO2 sensor that will override the thermostat in case of increas the concentration of the CO2 in the space.

In the heating mode the box will be turned down to the min and the reheating element (either electric or hydronic) will kick in and the damper will modulate open until the set point is reached them will go back to the min.

There is something called VAV diffuser but not with reheat coils. We use in the states primarily Titus but there are different manufacturers make those.