If you feel morally obligated to your friend and the beneficiaries really need the money, I would only accept the executor role if all legal heirs and beneficiaries explicitly accept the last will and indemnify you from any claims. Further, you need legal advice in the UK and Thailand for ensuring compliance with respective (tax) laws.
Advice for any future last wills: try to keep the assets separated by country, distribute the assets to beneficiaries in the same country, nominate executors for each country and notify your executors while alive.