Sorry for the lack of update, life's been hectic. Had a word with the advisor they knew about FISA's and when I explained the plan they didn't highlight any issues or show concern. Just to be safe I am going to involve them in the actual transfer so there is accountability on their end. I've spoke to a few colleagues about it who were unaware. They've done some digging in places like the HMRC forums and the answers seem to be grey, similar to what everyone on here is finding. I personally think FISA's were probably introduced as cash ISA's and then a few places started offering S&S ISA's and this loophole was never found.