Hi , from the wording of your query , I take it you're not technical ?
There are many different types of coaxial cable ; I know the external cable from the omni box into the house is RG6U , which is a 75 ohm cable . While this may share the same impedance with the indoor white cable , which may also be RG6U ( I don't know ) it is an entirely different cable designed to do a different job . Other similar cables , used for video signals include RG58 ( 50 ohm ) RG59 ( 75 ohm ) amongst numerous others . All of these cables have different construction and electrical characteristics ; if you use the wrong one it will not only make your own installation sub optimal , but the problems can reflect back down the transmission line and affect other people nearby . I know about these things because I both did electrical and electronic engineering at university , and worked in broadcast video for a number of years .
Oh and the cable you describe is terminated with F connectors ; do you know how to prepare the cable and terminate correctly for these connectors ?
If the answer is no , you should not attempt to do this yourself - it isn't like trying to fit a Belling-Lee connector onto a domestic TV aerial .
You are of course at liberty to argue the toss with VM that you feel the original installation was unsatisfactory and should be rectified FOC , but it is likely to take time , with no assurance of success ; the £25 call out fee for an engineer visit is quite modest and would see the problem resolved quickly and efficiently .