The UK Licence is a 3rd country ICAO licence so any credit towards gaining an EASA licence based on a UK licence are under those rules. The rules are available on the EASA website but in simple terms for example to gain an EASA ATPL(H) based upon a UK ATPL(H) you are credited:
- Flight training.
- Theory exams if you are within 7 years of when you passed your EASA ATPL(H) exams or the expiry of your last PC.
- You'll still need to do an ATPL(H) check ride and your UK ATPL(H) will need have a current PC.
- Crediting of R/T licence and English language seems to differ with authorities.
Austro were very good for me, but are not even replying to a colleague, so your mileage may vary.