It also seems that keep my current BT phone line is more "independent", but slightly more expensive
Acknowledge you lose advantage of internet + phone line/calls package deals PN does, but have you considered Primus (just for the line). A lot depends on your calling pattern as to best option, but for a simple
cheap line rental deal with no "included calls" (I just use 1899.com for the few calls made, and when I do call 01/02/03 it's a flat 5p, no matter if it is 5 minutes or 5 hours) then Primus may serve. No problem using PN as ISP (it's what I'm using now, submitting this post).
Yes, there's a minimum contract, but even during that you'd pay less per month (and be paying monthly) than the 'lump sum in advance' 'discount' deals that BT and others offer.
I don't know what the minimum fee is nowadays - possibly around 8 quid a month, but it does get mention on MoneySavingExpert.com and elsewhere (though obviously you will want to avoid any
phone line/calls + internet deals they are currently pushing).