If you break into the ring final circuit, put a Fused Connection Unit into the circuit. This will allow you to run anything up to 13A (approx 3kw) safely. BS7671 allows for 1no socket outlet to come directly from a ring final circuit without the use of an FCU, however this may already be in your ring circuit, to be safe run the ring into the FCU and take your Radial spur from that.
If you break into a ring, make sure the new socket is wired within the ring. That would be the best way of adding a new socket.
You can spur off a ring main one single or one double socket without adding an FCU and you can still plug in your heater as long as it has a three pin plug. The fuse inside the plug is there to make sure that the current won't go higher than 13Amp.
The third option would be to have an FCU spurred off the ring main and then on the load side of the FCU have a single socket fitted, where you can plug your heater.