Drivers are usually allowed to stop to load or unload. If the items are heavy or bulky or if the driver has a large number of items which would involve more than one trip, the car is allowed to wait on the yellow line. But the car should be moved and parked legally when the loading or unloading is finished.

Commercial vehicles are allowed to collect and deliver goods. If any paperwork needs to be checked, such as delivery notes or invoices, the time this takes can be included in the loading/unloading time allowed. Obviously, the vehicle must be moved after the delivery or collection has taken place.

 Loading is not permitted at places where the parked car could cause an obstruction, such as within 10 metres of a junction.

Sometimes, within a parking place, there is a yellow line called a "loading gap". The same rules as above apply to this yellow line.

Often there will be yellow ‘blips’ on the kerb. These warn that there is a loading restriction. Two ‘blips’ mean no loading ‘at any time’. One ‘blip’ means that loading is restricted at certain times, as shown on a white plate. Even Blue Badge holders with badges and time clocks are not allowed to park where there are loading restrictions in force.

source:.parking-appeals.gov.uk