Hi achitophel,
Thank you for reaching out to us in our community ands welcome back, it has been a while hope you are well, we can help port your BT number in no problem however you would need to make sure the number stays active while we take it and if possible cancel the disconnection, once we request it the process can take 10 Days, if the service cancels before this the number may be lost, we understand this is something you would like to avoid as you have had the number for 18 Years.
I will be more than happy to help further and raise the request, I will send you an invite into a private chat, once received please click on the purple envelope to accept.
Regards
Paul.