As has been said, if you have the virgin hub in modem mode then none of its other settings are applicable, it just passes all traffic straight through to its first LAN port. So any network features that you might want, such as DHCP, need to all be configured in the BT hub which is best left at its default 192.168.0.1 address.
I recently went the other way, from BT to Virgin. And even when virgin is running at half speed its twice as fast as BTs top speed - it all varies according to where you live, etc, etc. But Wifi problems are more usually down to building layout and nothing to do with provider choice. Mine were solved by a Wifi repeater the other side of the houses central chimney stack. Also obsessing with the provider speed is slightly pointless since even sitting a metre from the hub, using wifi is slower than any physical connection, ie. its an immediate compromise on speed.