My ISA transfer went pretty well! It took 8 working days without any issues.
I sent an email to ISA@trading212.com with their form filled as explained in https://helpcentre.trading212.com/hc/en-us/articles/360007081097-Can-I-transfer-my-ISA-from-another-bank-or-broker-
That was sent on Monday 3rd. Today (13th), the ISA transfer has been completed and my funds are in T212 (transferred from Barclays). So it took 8 working days.
Just sharing my experience in case is useful to anyone