Part M of the building regulations help with guidance on the recommended height of Switches and Sockets for new buildings to help with access and should be located so that they are easily reachable. The regulation also suggests that in habitable rooms this will be achieved by mounting the switches, sockets etc between 45cm (bottom of socket height) and 1200mm (top of switch height) above the floor level. Unless the dwelling is specifically designed for a person with limited reach, these requirements do not apply in garages and kitchens.
The Part M regulations also do not apply to replacement switches or sockets or to new switches and sockets in extensions or a dwelling which is being materially altered unless the dwelling previously complied with Part M. There is however, nothing to prevent you from applying these regulations in your own home should you so wish. Any alteration to an existing dwelling must not make it less satisfactory than it was before.
Something to note for convenience and safety is that any sockets positioned above a work surface, for example, above a kitchen work top, it is suggested that it should have a space of 15cm from the bottom of the socket to minimise the risk of damage to the cable/flex.