Even if I had moved both broadband and phone to BT retail, TTB still wouldn’t have charged me anything as they confirmed to me. I believe it’s only the TTB resellers who charge hefty fees for migrations back to BT.
I wasn't suggesting Pulse8 charged. They didn't.
There is a BTW charge for migrating an LLU phone line; essentially the same cost as a new line.
What the ISPs charge varies: When I from Pulse8 to Aquiss, the fee was £72; if I'd moved to IDNet it would have been £90. Some others, e.g BT just charge a flat rate fee for migrations. Perhaps your PO line was the same.
Ultimately the ISP will be paying for a new line install and if they are not charging you, they are subsidising it from their monthly fees.