TV aerial co-axial cable will never be suitable. It will have too high a resistance and signal drop and being unshielded is at risk of other interference.
If you must Do It Yourself then you need good quality satellite type cable. Even so there are many different grades of this. A
suggested something like
http://www.maplin.co.uk/Module.aspx?ModuleNo=275 might be appropriate. You'll need to add end fittings to that and they also need to be of good quality. Virgin Media's are all crimped on rather than the DIY screw on style. Bottom line if your cabling and jointing is not of sufficient calibre you will get signal loss and interference. You can even potentially disrupt the network serving your neighbours if its really bad. With the sort of length you are talking about there are risks that there will still be a drop that needs other adjustments to the signal strength but that will only be able to be adjusted by VM.
Bottom line the £99 VM fee might sound a lot, but if you must move the modem, and can't live with an ethernet extension from it, then the VM install could prove the best value.