I am a former BT line and phone calls customer and switched over completely to Plusnet - my ISP since Feb 08 - in January 2014. Despite being an existing PN customer they did me a good deal on my existing ADSL2+ BB which reduced the monthly fee (for BB) to £2.50 for 6 months instead of £9.99.
Then, last month I upgraded to Fibre Optic. PN advised me that as my line couldn't manage the 76Mbps download, there was no point me paying extra for that (how many ISPs would be upfront like that?) and so they set me up on the 38Mbps download package. Given that they had introductory packages for new customers I asked what they could do for me. To my surprise they were very obliging and said as I was prepared to sign up for a 2 year contract, that they would do the Fibre Optic deal for just an extra one penny per month! Installation was free, BT Openreach supplied the modem free and I only had to pay £5.95 p&p for my otherwise free Fibre Optic Router.
My current package is:-
Plusnet Anytime Calls + Caller ID (=extra 99p), + BT Sport HD (£7.49/mo) + Fibre Optic Broadband (up to 38Mbps), all for £23.48/month + Plusnet Line Rental, which I pay annually via Line Rental Saver. That's also cheaper than BT's LRS product.
Apart from some slight drop off at busy times of the day I often seem to get the full 38Mbps down (sometimes slightly more) and 18.5Mbps up.
I have always been happy with Plusnet as an ISP and my opinion hasn't changed since I switched the telephone calls/line over to them as well.
So I would suggest asking if they can do a deal for you and see what they say.
PS: The only real fly-in-the-ointment with Plusnet is that they stil use Ofcom Market Areas to determine what customers pay, so if you are unlucky enough to be somewhere where BT is the only other provider, there will be a premium on the advertised price.
More info :
Plusnet Market Rates