If you are on TalkTalk's LLU product ie. including "free" broadband you no longer have a "BT" line. TalkTalk have connected you to their own equipment in the exchange.
To receive another ISP's product you need a "BT" line ie. connected to BT wholesale's network, not necessarily back to BT retail for line rental.
Some providers can arrange this, it's merely an administrative procedure between the voice provider and BT Wholesale/Openreach.
Whoever you get to return your line to BT's network will tie you in for line rental but you should be free to choose whichever broadband provider you wish.
The phone co-op is one who can convert from LLU back to BT but charge £40, BT retail I think should be free. Probably others out there who can do it.