I went with BT (when I moved into my flat) because I was keen on the Standard Line Rental deal. It was perfect because I made very few calls and these never exceeded my call allowance so I paid nothing beyond the line rental. They told me that I could be hooked up without the need for an engineer to visit and did so days after I agreed to have a line. There wasn't the £120 fee as the line although not with any telecoms company, was still connected to the exchange and she was able to tell me that over the phone (I was calling from the office as I hadn't moved in yet). On a subsequent call for someone else the lady said that if the line had been disconnected anywhere between me and the exchange then the fee would have been charged.

Basically if you have a dial tone and call 17070 you should get the phone number read back to you, If that is all you get then it's not a BT line. If you get some test options after the phone number then it's an active BT line. In either of those cases then there should be no charge for reconnecting the line, as long as the dial tone is still there when BT test the line.

If there is no dial tone then the £120 fee will likely apply. I also think that the price is set by Openreach (which is part of BT) and agreed with Ofcom, who have the final say.