We have put together a short survey to capture your feedback in terms of what BT can do when you experience home Wi-Fi issues. We would appreciate it if you could take a couple of minutes to complete a short survey. Read more from here, Fix my home Wi-Fi
You can use your residential line. It is only if you are running a business that you would be breaching the Terms and Conditions and even then unless it was a more than a one person home business I doubt BT would do anything about it.
Home working is on the rise and its not a requirement that you have a business line for that purpose. BT users who are running a business from a residential line have agreed to the terms for a residential service so if anything went wrong with their line they cannot claim compensation for business use.
I occasionally work from home (for my employed job) do I need business broadband or can I still use my residential service?
This gives the guidance.
Also Section 11b note xii
Homeworkers, who are employees of a company, are normally not classed as business users.