A few misconceptions here, arising mainly from sloppy terminology. The UK PPL does not expire and, therefore, never needs to be revalidated or renewed. All that is required is to renew the SEP class rating (or any other class or type rating) attached to the licence. The UK CAA has determined that the revalidation requirements for ratings/certificates attached to UK licences will be the same as for EASA licences
Refresher training, at an ATO, is required sufficient to reach the level of proficiency necessary to safely operate the relevant class or type of aircraft. The amount of training will be determined by the ATO, which will issue a Course Completion Certificate when the necessary level of skill has been achieved. An examiner may not commence a proficiency check unless the candidate holds a valid course completion certificate.
Under current regulations, the UK PPL will be valid on EASA aircraft until 8 April 2018 with LAPL privileges and, thereafter, will be restricted to Annex II aircraft (e.g. ex-military/ vintage/ homebuilds/ etc.).