A new regulation was brought in with the release of the 17th edition of the wiring regulations in 2008. This placed requirements that all circuits with cables buried less than 50mm from the finished wall surface shall have an RCD for additional protection.
In domestic properties, almost all cables which are buried in the wall will usually be less than 50mm deep. Particularly on internal walls, most are not 100mm thick so if not less than 50mm from one side, it will be from the other!!
The regulation for RCD on cables less than 50mm from the surface was brought in to protect buried cables against damage. One example would be banging a nail through a wire when hanging a picture. Another example would be drilling through a cable when attempting to install a shelf.
There are some exceptions to this requirement, however these would be very rare on a domestic installation. One is having earthed metallic protection in the wall in front of the cable. The other is using SWA (Steel Wire Armoured) cable, again not something you would see buried in a house wall!
Obviously houses built or rewired after 2008 won’t usually be affected by this problem. These homes will have been built with a more modern consumer unit with full RCD protection of all circuits.