Assuming its a standard cavity wall, 100mm external leaf, 100mm cavity (with 50mm cavity insulation) 100mm internal leaf.
The cheapest way to insulate the wall is to use "blow-in" cavity bead or fibre insulation.
Internal dry-lining would be more expensive, using an insulated plasterboard slab. Services such as radiators, wall sockets & light switches would have to be moved out the thickness of the board, on all external walls (3 no. walls in a semi-d.)
The most expensive way would be to insulate the walls externally. Cills, down-pipes would have to be adjusted, the thickness of the insulation.
SEI offer grants for all three types of insulation. You will have to insulate the attic too.
Always get 3 written quotes from reputable installers for the works - see SEI site for list of contractors n your area.