The only way to get any sort of accurate figure is to contact the relevant gas
distribution company. Confusingly, that's not the same as any of the various gas
supply companies.
Take a look at the map here:
http://www.nationalgrid.com/uk/Gas/Connections /domestic/
If you live in any of the dark blue areas, your gas distribution network is provided by National Grid. Click on the relevant area and then click on 'New Supply Pack'. The page you get to has lots of information, including links to download the appropriate paperwork which will enable you to obtain a quotation.
If you live in an area shaded light blue on the map, click the relevant link (on the right of the page) to be taken to the website of the relevant gas distribution company, where you should find similar information.
Note that the work done by the gas distribution company only provides you with a gas pipe. You also need to contact a gas supply company (such as British Gas, Powergen or UK Power) to install the meter and finalise your supply.
Chris