All UK directory enquiry services (whether that be the BT Phone Book or companies like 118 118) use the OSIS database, which collates data from all landline providers and it's only those providers who can authorises changes to that database.
By default all subscribers numbers are included in the database. While you were with BT your number was withheld from the database (because you'd instructed them to do so) but when you switched to your new provider you failed to tell them to withhold your number, so the default situation (where your number is automatically included in the database) applied from that date.
So it's not BT you need to be contacting. It's your current landline provider, as they're the only people who're allowed to change the database.