******** used to sell an option for a 1 off payment of £10 called dual IP. With this they provided a network switch and reconfigured your modem to provide a second DHCP address.
Unfortunately the merger with ****** killed this. If you got it back then, you will still have it. and if you didn't you can't have it. Calls to customer services confuses them, they think there is something wrong. They have in the past removed it and deny any knowledge of the product when you demand they re-instate it etc.
The modem config file was set up to allow the modem to allocate two distinct IP addresses to two different user devices (ie PC and games console). As mentioned above, you needed a switch (or a simple hub) to allow these multiple devices to connect to the modem.
From what I remember, each modem is actually capable of handling between 8 and 32 different IP addresses (depending on modem model). The number of IP's is selected in the config file.
The config file also specified the max speed of the modem. If its 10Mbit/sec then that 10Mbit will be shared between all IP's. Its not 10Mbit for each IP !
For most purposes, a NAT router will give you the functionality of multiple IP addresses whilst only actually requiring one external IP.