The requirements for converting existing BGA tugpilot qualifications into EASA ratings are explained here,
sections J & K at the bottom of the page. (This page was written some time ago, before the extension to the derogation; references to 2015 should be read as references to 2018.)
Basically:
- have qualified as a tugpilot at a BGA club
- have at least 30 hours & 60 TOs & landings PIC in the relevant class of aeroplane (SEP or TMG)
- 10 flights tugging as PIC at a BGA club
All you need to convert is to find an appropriate person (either CFI, club chairman or club secretary) to certify on your application that you meet the requirements.
The CAA & BGA have pragmatically agreed that pilots may continue to operate to the old, pre-EASA rules, even if the pilot holds an EASA licence, at least until Apr 2018. You can still fly as a tugpilot without holding an EASA tow rating, a tugmaster or any other pilot may continue to instruct potential tugpilots without holding an FI or CRI certificate.