Can I use a junction box to extend a ring main?

The best way of using junction boxes to extend a ring circuit, is to find the length of cable between the two sockets on the existing ring closest to the desired extension area. Once located it can be cut in the middle giving two lengths of cable from the existing circuit to use in the extension.

Can I spur off a ring main with a junction box?

The easiest way to fit an extra socket to an existing ring circuit is by adding a spur. A spur comes off the ring circuit via a junction box, or more usually via an existing socket - which ever is easier.