The same handset can be used with copper and the BT FVA ( VOIP) service. The is an ATA ( Analogue Telephony Adapter) in the box that you plug the phone into.
The difference in cost is because the phone service only costs a couple of quid a month! The majority of either Phone or BB cost is the infrastructure cost to get it to you! and this is the same for one / other or both. The exchange and core costs are not high as for VOIP it is only 64kb within a large Mb network plus a call control ( computer with routeing software) and for PSTN it is practically all depreciated so OFCOM allowed costs are small.
FVA VOIP service will only work whilst you have power ( including Battery Back-up) while PSTN Copper will work as long as the exchange has power (including Battery and standby generators) otherwise they are indistinguishable. Native VOIP service should be the same quality as FVA BUT can differ if there is congestion anywhere on the routeing. ( Depends on the VOIP priority on each and every route the call takes).