As well as the cabinet may be full as mentioned in the above post Openreach have started moving people onto Fibre internet as opposed to copper based services.
Now broadband do not ( currently) offer Fibre based internet service products, only copper based products that are being phased out .
The way people are being moved off copper and onto Fibre is by the means of openreach declaring all copper based connections on an exchange to be "Stop sell" this means internet service providers will no longer be able to sell copper based services, only Fibre
For the end user such as you and me, this means that you will no longer be able to move to another copper based provider, only Fibre , and as now dont do fibre then you will not be able to move to now.
See here to see if your exchange is on the list: