The longest pre-made cable you can get from VM is 3 metres. If you want anything longer you will have to book a non fault technician visit at a cost of £25.
Bear in mind that VM uses specialised cable that is not sold anywhere else. The cable sold on eBay & the like is not approved, no matter what the seller claims. Any interference entering the local network as a result of DIY alterations could cost you a considerable amount of money. Also making alterations to the amount of cable affects your signal levels, & hence service performance. The technician will make up the cable on site, & then if necessary adjust the signal levels in the street cabinet outside.
To book, just call the normal faults line (150 from a VM phone, 0345 454 1111 from others), or if you are prepared to wait a few days a forum team member will respond here & set this up for you.