I stumbled across this article in The Guardian https://www.theguardian.com/money/2017/feb/22/bt-refuses-refund-prepaid-line-ren...

After The Guardian took it up with Ofcom.BT accepted its refundable.

"No, what you have been told is wrong. We asked the regulator, Ofcom, about your experience, and it says: “We contacted BT to express concerns about the exit terms it was imposing on customers who had paid line rental upfront, and BT has now assured us that customers wishing to end their contract following its price increases will now be given a pro-rata refund of any line rental paid upfront.”
"BT has confirmed it accepts this and will be contacting customers and briefing its advisers. To be clear, if you leave a phone company mid-contract due to a significant price rise within 30 days, you should not incur penalty charges – including pre-bought line rental."

Which should  mean that all Pre-Bought Line Rental whoever the supplier is, is refundable