Before installing a new socket outlet, you will need to consider where it is to be positioned and how the cable supplying the power will be routed to it. The former will encompass your requirements as well as those of the wiring regulations. The latter will depend upon how you intend linking to the existing ring main and the type of surfaces the cable will need to pass.
Regarding the provision of power, there are three basic ways this may be done.
You must comply with the current regulations.
Ring main extension
The ring main may be extended, subject to the requirements of the regulations, to incorporate the new socket.
Spur from socket outlet
You may take a spur from a socket on the ring main provided it does not already have a spur, and the regulations regarding the number of spurs on a ring is not contravened. Make sure that the socket outlet in question is not already a spur itself.
Spur from ring main
You may cut into the ring at some point as near to the new socket as possible and fit a junction box to feed a spur. Again, be sure that you comply with the regulations regarding electrical installations. Where a junction box is not accessible for inspection, maintenance and testing, a maintenance free junction box must be used.