I'm not sure why you would expect DVLA workers to be spotters and I wonder how good they or any other average member of the public would be at recognising a C130 head-on and possibly in silhouette.
Swansea, as a Town with a population of more than 10,000, is a town avoidance (see
www.mod.uk/issues/lowflying), so, presumably, it would be unusual to see a military transport ac flying there below 2000ft. If the DVLA building is, say 300 ft high, and the ac was '400 yards' (okay, make that 800 yards) from the building, then one could assume the ac was flying well below 2000ft, within the town avoidance. If the crew really were avoiding weather, fair enough, but I think an apology was in order, in the circs.
I doubt slagging-off DVLA helps the cause of the aircrew involved. It might give people the wrong idea.