I have peered up at various posts to see if there are CCTV▸ cameras on the tops, but haven't spotted any. And there aren't any signs reinforcing the prohibition of parking. I guess the lines are meant to drop a broad hint and may not be enforceable in law. I park on them for a few minutes at quiet times to pick up and see off friends, but they are ignored by many others in the evening rush hour.
Marlburian
Setting down and picking up passenger is allowed almost anywhere - yellow lines don't forbid it. Likewise loading and unloading, unless there are the kerb markings that indicate restrictions on that. (I note, however, that supermarket and parcels delivery drivers are unwilling to use this right to the full - they'd much rather sneak onto any off-road parking they can spot, whoever's it is.)
But you have to be doing what you stopped for - not just waiting to do it. The rules about on-street parking and the yellow-line restrictions are all worded is terms of "waiting", not "parking". Of course there are other higher-priority bans that do apply - motorways, red routes, etc.