On BBU OFCOM mandate a minimum of 1 hour in standby. Last I heard the BT BBU gave a minimum of 4 hours standby. ( Both voice only NO Broadband). Both will be shorter if the phone is in use so maybe 15 min and 1 hour talk time.
The rental charge for the phone service is in the £1-£2 per month range at present, the rest is paying for the line. The pair rental, basically the same for voice only or BB only, around £80-90 per YEAR I cannot see this changing. The rest of the charge is for the BB and ISP service whichever level you pick.
The (Wholesale) IP voice service is likely to be also in the £1-£2 per month range but may include calls in this as the bandwidth is minimal compared to the BB network size. ( You are paying for the call server, ATA etc and Number costs. OFCOM charge for each number).
I can see national calls being 'free' as this reduces/removes the cost of all the complex interconnect payments and billing although this will depend on OFCOM allowing this! ( They have set terminating prices to nearly zero at present but that may change) The service will have to compete with SKYPE price so have to be very cheap.