Life time PPL
It is what it is - a lifetime PPL. You need do nothing. Do not confuse a Licence with a rating.
If you have a current SEPL, MEPL or other rating that is current it is so whether using the UK national licence or the JAR complient one issued by the UK. In the event that you did not wish to renew your UK/JAR complient licence but at the time it expires the relevant ratings are valid then you continue flying subject to your UK nationaly issued licence. You simply take the validation page out of the JAR licence and insert it into your UK National PPL.
Should your class rating/s have expired then as with all cases you renew them by a simple class Renewal Test in the the case of the SEPL. The MEPL rating must be revalidated/Renewed every 12 months anyway.