OP is significantly changing the length of cable so it will need signal adjustments/balancing by the Tech - possibly in the Street Cab. They are not charging for the cable per se rather for the tech call out and time to do the work. Ask how much a sparks will charge you for moving a socket or rewiring a light fitting.
My daughter once bought pre-made and "guaranteed VM compatible" cables from Amazon to move her V6 box. All it did was ruin the two neighbours signal with noise ingress - whilst her signal was fine (she said). The VM techs discovered this when they checked our system to try and pin down the source of the neighbours' noise. They spotted it instantly and swapped it for one of their cables and the neighbours problem disappeared.
I was just thankful they didn't charge us a large sum for breaking the T&C's of our contract - which specifically forbids what is being suggested - which they were well within their rights to do and add to our bill.
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John
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My services: HD TV on VIP (+ Sky Sports & Movies & BT sport), x3 V6 boxes (1 wired 2 WiFi,) SH2 in modem mode with Airport Extreme Router. On VIVID200, Talk Anytime Phone, x2 Mobile SIM only iPhones.