Loading is allowed in many (but not all) places where parking is not normally allowed. So, what counts as loading?
The legislation that creates parking restrictions does not actually define loading, so there is no clear answer. One of the most detailed examinations of what does and does not constitute loading is an adjudication decision by E.J.W.Houghton Jane Packer Flowers and others [PDF]
The below are things that affect how likely something is to be considered loading, but none provide a definite answer by themselves:
Size of items to be loaded
Is the vehicle being loaded, or is the vehicle being used by a person who is carrying an item? Collecting cash from a cashpoint or bank for the driver to use would normally not be loading – the person is collecting the cash, not the vehicle. Collecting an item too big or heavy to reasonably carry is more likely to be loading. A courier delivering many small packets, though, is likely to be loading even though the individual items may be small enough to carry.
In the course of a business
As in the examples above, is the driver or the vehicle making the collection? Collections or deliveries in the course of a business are more likely to be using a vehicle because there is a need to, rather than it just being convenient, especially if it is part of a series of collections or deliveries.
The wording allowing loading usually limits the time to “for so long as may be necessary” (or some similar wording), and vehicles in places where loading is allowed will normally be observed before a penalty is issued. Time to ensure frozen goods are placed in a freezer, or a delivery note is signed would be covered; time taken choosing items in a shop, queuing for the till, then paying, would generally not be considered a necessary part of loading.
This makes choosing an appropriate observation time a difficult balancing act – stopping on a yellow line to buy a paper from a newsagents may only take a minute or so, but can not really be considered loading; the same newsagent receiving a delivery of cigarettes from a car parked on the same yellow line may reasonably take more than a couple of minutes to check the quantities match and sign the delivery paperwork. Longer observation times will require more evidence from motorists to justify that the time taken was necessary, and will reduce the chance of penalties being issued where genuine loading was taking place – but it will also contribute to the myth that parking for short periods of time is allowed.
This does not, strictly speaking, measure whether or not a vehicle is being used for loading or not, but more activity makes it more obvious to an officer or warden that loading is taking place, and so reduces the chance of a penalty being issued in the first place.