Waiting, Loading and Parking are all ill-defined in most texts. But loading specifically requires the activity - and the vehicle - to be 'in use'. Waiting is different from Loading because the vehicle is not in use. Waiting is also different from Parking because the driver is required to be in the vehicle and the vehicle's presence requires a purpose not related to the driver. Using a loading bay for 'popping in to the shop' or 'just making a phone call' is not Loading or Waiting, it's Parking.
But road markings also dictate whether you can stop or not, and wait or not, or load or not. Loading that's prohibited for some period of a day or week or month gets a single bar across the kerb. A prohibition if more than four months gets two bars.
Edit: TSRGD says:
"6.3 Standard exemptions to a waiting prohibition are not shown on the signs. These exemptions include stopping to pick up or set down passengers, and causing a vehicle to be stationary for the purposes of loading or unloading goods from that vehicle.
Loading generally refers to commercial loading or to objects that are too heavy or bulky to be carried very far by hand, but does not include time for purchasing the goods.
Where loading is not permitted, additional signs are provided. These have a white background, to contrast with the yellow “no waiting” signs. Disabled badge holders are permitted to park for up to three hours where there is a waiting, but not a loading prohibition. Where a length of road is not suitable for parking by disabled badge holders, a prohibition of loading should be considered. However, if a loading provision is necessary, a loading bay should be provided as this cannot be used by disabled badge holders.
A prohibition of loading still permits drivers to pick up and set down passengers."