you can buy the cable and connectors at your local maplins store. some stores will offer pre-made cables but i think you'll find they r upto 5 metres
i believe this is what u need but CHECK with someone at your local store.
F TYPE CONNECTOR PLUG
http://www.maplin.co.uk/search.aspx?...2m10&worldid=2
these plugs never work as they should. they dont screw on properly or crimp properly. the metal either breaks or is not tight enough around the cable. i used some cellotape to keep plug securely on. works fine for me.
RF COAX CABLE ( not sure the exact one you need, ask local maplin store)
http://www.maplin.co.uk/search.aspx?...2m10&worldid=2
[ just noticed you said this is for a STB and not cable modem. don't know much abt STB's so not sure the advice above is applicable]