We had to fit gas in our house 12 years ago, it cost about £400 and British Gas did it. There was already gas in the street, just for some reason not in our house. It would have cost more if there had been no gas in the street either.
Then we had central heating and a shower fitted, which was 5 radiators, a boiler and a shower. It cost £3500. But like a said, that was 12 years ago. In the midlands.
I think Transco did it for us. I'd suggest you phone whatever company you'd like your supply from and ask them about it. If others have gas in the street, some will probably have big white boxes on the side of the house. We had a full written quote and they made good the drive (mainly paving slabs though) and re concreted over that they had to dig up. From memory it was really easy to sort, but (on a funny note) I was surprised to find them rustling around outside our house with a 6.30am start!
there are fixed prices depending on how far it is from the pipe to your front door. Property in Hampshire £1500 to connect from street. NIGHTMARE to organise with utility company and the actual company that does it. Maybe worth changing suppliers to get it done quicker ie Bgas had 3 month wait.
We had ours done last yr. Anyone can dig the trench for you - we had a labourer come to dig our trench. He only charged £7/hr. Then national grid laid the pipe and the labourer came back to fill the trench.
I called our power company first but they just directed me to national grid. They'll be able to confirm if there's gas on your street.