Faxing over Voip can present some challenges which require increasing the bandwidth (G711 codec). This is configurable on routers supplied as part of the BT's Broadband Voice service. The transmission speed of the Fax is also a critical factor and this needs to be slowed to a speed of no more than 9600 bauds. The configuration of the fax machine will be dependent upon the manufacturer’s own set up of the machine, so having the fax user manual to hand when setting up the service is often an essential part of the process.
As you can see, it is certainly possible to use Voip to send and receive Fax's but as you rightly suggest using a Fax to email service seems by far the simplest way forward.
The good news on this front is BT is likely to be making an announcement in the next few weeks about this type of service. We will keep you posted.