As Spudqueen's post indicates, very few countries actually have a '6 month validity rule'. You can travel to anywhere in the EU right up until the last day of your passport's nominal validity. Most other popular tourist destinations will also accept your passport throughout its full (nominal) validity. (Even the USA, which has some of the strictest entry requirements in the world, waives their '6 month rule' for British passport holders).
There are only a few countries that require your passport to have a certain period of validity on it when you enter (or plan to leave) their country. Unfortunately a few travel agents can't be bothered to train their staff properly, resulting in all of their customers being told that they must have 6 months left on their passport. This has led to a myth that you can't travel anywhere outside the UK without 6 months left on your passport.
As both Barry1010 and Spudqueen have stated, if you renew your passport prior to its expiry, any unexpired time (up to a maximum of 9 months) is automatically added to the validity of the new document.
Chris