2. How can you PROVE that it was your own money? Even if you withdrew the cash from somewhere else - you cannot prove that when you paid it into his bank, it was the same money. Here, even your completed ISA or any other evidence does not PROVE that it was your money.
first of all, it also sounds fictitious to me, even if it's true, you gave the money with your own will, so either you go to English, Polish police or not, there is nothing to do. There is no coerce
Pauline if you have been made a transfer from your account to his with the intention of using a joint ISA, then you have acted in good faith.
1. Report the theft to your local Police, they are obliged to investigate.
2. You have his bank details so go to the bank and request a hold/stop on the account because of illegal activities. The bank will need proof there has been an illegal activity.
The bank will co-operate with you once they have been informed, they can even inform the bank in Poland if he has sent the money to Poland
If he has cleaned out the bank account in cash, you have a much more difficult case.
When you sent the money to his account what title did you give the transfer?
Did you write down loan, joint saving, ISA?