What are the regulations covering this?

BS7671:2001 Requirements for Electrical Installations (IEE Wiring
Regulations Sixteenth Edition).

Height of sockets AND switches governed by Building Regulations part
M - to be installed between 450mm and 1200mm above floor level, though
there are excemptions for rooms such as kitchens where sockets are
installed above worksurfaces. Part M concerns itself with "access", e.g.
for disabled persons, and also insists on things like "level" access to
the main entrance, and a wheelchair-accessible downstairs loo in
newly-built or renovated houses.

This applies to new buildings only. You can place the sockets at any height
you wish when doing a rewire as long as they can be used safely (i.e. not
usually on skirting as it causes the cable from the plug to be under
stress).

Adam