The installation of a new phone line can cost around £130 with BT so you are not paying for that. If you were promised more "free" items then you either accept the bill and pay or you dispute the bill and refuse to pay.
As you do not have a written record of the conversation with the salesperson or the call centre, it would be sensible to put the situation in a letter to Virgin so that it is all properly recorded in black and white. If you don't, you will find that Virgin "are unable to find a record of that conversation". It is not uncommon for salesmen to make promises that will not be kept if they believe that they can make a sale. If you do decide to dispute the bill, you should make it clear to Virgin as soon as possible that you are doing this and you should ask them to either cease the service immediately or charge what was promised by the salesman.
If you signed an agreement with details of the charges then that would normally take precedence over anything that was agreed verbally.
It is possible that Virgin could involve debt collectors and/or the courts if you refuse to pay so it is very important if you decide to go in this direction that you keep copies of letters and obtain (free) posting receipts or use recorded delivery.