There is no measured limit defined in the regulations AFAIK, but reproduced below is a small section of the NICEIC technical manual that has been touted around before when similar questions have been asked re proximity of sockets to sinks:
2. Domestic premises
Domestic kitchens, bedrooms and cloakrooms (unlike bathrooms and shower rooms) are not included amongst the special installations or locations in Part 6 of BS 7671 and are not rooms where the resistance of the body is likely to be significantly reduced. Therefore, normal precautions against electric shock should be adequate and the general rules of BS 7671 are applicable.
However, whilst BS 7671 does not forbid the installation of a socket-outlet or other accessory close to a sink in a domestic kitchen, bedroom or cloakroom, the requirements of Regulation 512-06-01 have to be taken into account.
The requirements of Regulation 512-06-01 mean that ordinary BS 1363 socket-outlets and similar accessories are not suitable to be installed so close to sinks or draining boards that they are likely to be splashed with water or operated with wet hands.
The penetration of water into an accessory could have serious effects. Water entering the terminals and mechanism may lead to corrosion, internal arcing, general degradation and eventually malfunction. Additionally, and more seriously, water may provide a track for the phase voltage to be transmitted to the front cover of the accessory, giving rise to the risk of electric shock.
A rule of thumb recommendation for domestic premises, to avoid the effects of splashing, is that ordinary BS 1363 socket-outlets and similar accessories should ideally be mounted not less than about 300 mm (ideally not less than about 1000 mm), in the horizontal plane, from the extremity of a sink top or wash basin.
Where splashing of accessories cannot be avoided, equipment having a degree of protection of at least IPX4 (protection against water splashing from any direction) or IPX5 (protection against water jets from any direction) is likely to be required. This type of equipment is, of course, unlikely to be visually acceptable in most domestic situations indoors.