Are you actually a BT broadband customer?
It's quite simple, instead of sending your domain name email via the BT email servers, simply send them via your domain host's email servers. Otherwise just add the the SPF information to your domain DNS record as posted previously.
You just need to ask your domain provider to add the SPF details
If the problem is the lack of SPF details, it is nothing to do with BT, it is your host that should be implementing the details I posted regardless of what they say.
Who is your domain provider?
Did you actually read the links I posted explaining how SPF works and it's purpose?
As asked previously, what client (app) are you using to send the emails and how exactly do you have it configured? Are you authenticating the mails with a btinternet email address?
This article may help your understanding
Prevent mail to Gmail users from being blocked or sent to spam - Gmail Help (google.com)