Hi,
I can only advise on knowledge that I have but to my understanding you will need to work within EASA 145 whilst completing the OJT, this OJT will be recorded in a specific OJT booklet produced by the company or training organisation and approved by CAA. You will have to complete min 50% of each ATA chapters' tasks with a good spread of work. I believe there to be a min amount of time to complete the book (6 months?) check the regs.
I know Connies who have had to go Permy so they can get this out of the way, so it looks as though it could cause problems for many working outside easa.
Whilst not directly answering answering the question of where, maybe this info will better guide to who to approach. I know BA, TCX, Mon all have approved OJT books for 320