Everyone in London and major cities knows about CPZ (Controlled Parking Zones). These are areas where certain rules/regulations apply. Here is a common definition.
A Controlled Parking Zone (CPZ) is an area where parking is only allowed on certain parts of the road for a limited time unless you have a permit. There will be signs on the side of the road to let you know how long you can park for without a permit.
Both the British Parking Association and the International Parking Community have instructions in their Codes of Practice about what one of their members needs to do.
The International Parking Community expresses this requirement as
This is the sign they say should be there but is frequently missing as it tends to reduce the number of tickets they can issue. It also is a basis for claiming that drivers are "ignoring the signs" as self-justification for them being there. Drivers don't want to pay £100 and will park elsewhere IF the signs are clear.
Example of locations with "missing" signs
There are two sites in London operated by Parking Control Management (UK) that are clear examples of the predatory practice that the IPC claim they are against. The first is Graham Park Way covered by the Parking Prankster here. But a second has emerged at Ruby Way Development in London NW9.
So if you get a parking charge go back and get pics of the signs at the entry to the zone to see how clear - or not - they were.