OK, the equipment may have been upgraded, but the premises which contain the exchanges, and a vast amount of the last-mile copper, all the telegraph poles, and the wayleaves agreements, and also many of the streetside connection boxes were in the original deal.
Not only has BT been able to use all of this without having to pay anyone, they have actually been able to achieve a capital gain on buildings which used to be exchanges, but have actually had the service consolidated into other nearby exchanges (think how much more compact digital exchanges are compared to the Strouger exchanges that have been shut down), and sold on. I was also told some years ago by someone in the telecommunications industry, that BT actually made money from ripping out the copper based long distance network, scrapping the copper, and replacing it with fibre.
If you were a new player, how much would it cost to put the last mile infrastructure in and buy the buildings for the local exchanges (especially in cities). We had an approximation when the cable infrastructure was put in 20 years ago (by the way, it was not Virgin, it was the small companies that merged to become NTL and Telewest that did the installation, and this is not Virgin Media by another name), and it was expensive then, and effectively bankrupted them. Think how much it would be now! (that's why they are experimenting with fibre through the sewers).