Is it necessary to connect a boiler and gas meter to the main fuse box?
I know you're in the UK, but I'll answer for the US in case someone there has the same question.

Boilers and furnaces must be powered by a separate, grounded circuit.

Any metal piping system that is likely to become energized must be bonded, including gas pipes, but the equipment grounding conductor run with the circuit conductors feeding a gas appliance "shall be permitted to serve as the bonding means":


  NEC 250.104
  (B) Other Metal Piping. 
  
  Where installed in or attached to a building or structure, a metal piping system(s), including gas piping, that is likely to become energized shall be bonded to the service equipment enclosure, the grounded conductor at the service, the grounding electrode conductor where of sufficient size, or the one or more grounding electrodes used. The bonding jumper(s) shall be sized in accordance with 250.122, using the rating of the circuit that is likely to energize the piping system(s). The equipment grounding conductor for the circuit that is likely to energize the piping shall be permitted to serve as the bonding means. The points of attachment of the bonding jumper(s) shall be accessible.