Yes, you can buy it. Satellite grade coaxial cable, with f-connectors at either end, is what you need. Also buy an adaptor allowing you to screw the f-connector on the end of your existing wire onto the f-connector on the coax you buy (from Maplins or wherever). It's safer to buy ready made-up cable than to try to do your own with your own connectors crimped onto the end.
Some will tell you that adding extra wire to your set-up breaches NTLs terms of service, and some will say that you can degrade the service received by your entire street by doing this, but TBH last time I had an install tech at my house he made a load of cable for me and left it behind, in case I ever felt like moving things around, so I wouldn't worry about it too much so long as you buy quality materials.
