This article explains how to wire a UK telephone extension.  You are allowed to install extension wiring so long as the connection to the master socket is via a plug, or in the case of the master socket with a detachable lower half, via the connectors on that section.  If you decide to connect directly to the master socket or install/replace a master socket you should be aware that you are in breach of your licence to use the phone service and your service provider may charge if they have to come to rectify a fault caused by your efforts (it is rare for them so to do unless you have been really ham fisted).

NEVER connect un-approved devices which take power from the mains to the telephone system.  Even if you survive the mains on the cabling the poor wireman sitting in a muddy hole some way away trying to find a fault may not.