|>>|| No. 26766
Tech guy here.
You should not fit one yourself. This is definitely engineer work.
As it is, don't try and do it with BT, they will charge you for this. You will not be their customer, strictly speaking, until you have a live service. So break out now.
Certain companies will cover the costs of the Openreach engineer to fit this for you. I know, for example, that the Post Office Homephone and Broadband is currently waiving engineer fees. If you sign up as a customer with them, they will fit the master-socket at no cost to yourself.
This could be done within three working days.
Quick correction on another poster. The internal socket also belongs to Openreach, just not the front cover of the socket, which belongs to the customer. So, you cannot fit it yourself, as it is not your property.