All the RRO calls for is for the assembly point to be "at a place of total safety" but it does not specify any distance.
Under the old Fire Precautions Act a place of ultimate safety was cosidered to be a place where people can disperse in open air and proceed a safe distance away.
Hence in my view an assembly point could be next to the building, provided people can move away from that point, in safety. So a point next to the building where people can walk away from the building across a road or a car park etc. would be safe, however a point in a courtyard, for example, where people would have to go towards the building to get away from the building ie through a gate next to the building would not be safe.
In short like everything else do a risk assessment.