From what you say, your line rental is tied to BT Consumer for a while. How long?
In terms of the connections at the exchange, your choices are between a BT Wholesale-based ISP using the same exchange connection, and TalkTalk or a TalkTalk Business Wholesale reseller with a different connection.
Some but not many ISPs can offer you a TalkTalk Business broadband connection with s "BT" line. TTBB has a good reputation.
I'm not sure where the Plusnet "£19.99" to upgrade comes from. Given your 14Mbps FTTC estimate and that exchange, that doesn't make any sense at all. Please could you give us more detail on what the quote is for?
Importantly - what is your BT bill per period for the line, and your Plusnet cost per month for the broadband?
The indispensable man or woman passes from the scene, and what happens next is more or less the same thing as was happening before.
My broadband basic info/help site - www.robertos.me.uk.
Domains, site and mail hosting - Tsohost
Connection - AAISP Home::1 80/20. Sync 59999/14372kbps @ 600m. - BQM