PAYG has always been expensive, even on the PSTN network. Charges are the same in both cases.
If you make a lot of calls, then the unlimited calls option is better value.
This is really a billing issue, as its not specific to Digital Voice.
Using BT DV isn’t compulsory, so if the PAYG rates are not to your liking, there is no obligation to use it , absolutely nothing stopping you signing up with a traditional VoIP supplier ,as the sole telephone supplier or in addition to BTDV , although it’s undoubtedly VoIP ,DV is much more secure than traditional VoIP services, plus a quick look at a traditional VoIP supplier that offers 1p a minute also have a £10 per month per number subscription, if you want that number to look like a traditional STD number , rather than the normal VoIP 0330 type numbers , adds another £1.95 , so in order to get the 1p per minute you also pay £12 a month subscription , BTDV is £2-£5.
It may be the VoIP provider you quote , that charges 1p a minute , may not be the one I checked , but they all charge something