No action is needed in respect of the licence which, as has already been stated, does not expire. To exercise the privileges of the licence, however, you need a valid type rating and so it is this that will need to be renewed (not revalidated).
To renew a type rating, you will need to undertake refresher training at an ATO (not FTO) or, until 8 Apr 2018, a registered facility in order to reach the standard required to pass the proficiency check. The amount of training will be decided by the ATO or RF who will not recommend you for the proficiency check until you have reached the required standard - without the training organisation's recommendation an examiner may not conduct the check.
Bear in mind that, once renewed, the UK PPL(H) will be valid on EASA helicopters only with LAPL(H) privileges and then only until 8 April 2018. Thereafter, the licence will be valid only on 'Annex II' helicopters (e.g. Rotorway & ex-military). To convert your UK PPL(H) to the EASA equivalent is quite straightforward - all that is required is to demonstrate, to a CFI or examiner, proper use of radio navigation aids. The training for this can be included in the refresher training for the type rating revalidation and competency demonstrated during the proficiency check. All that is then required is to make application to the CAA for the issue of an EASA PPL(H) (using Form SRG1104 or the equivalent on-line application) and to hand over the usual bucket of money. Have a look
here