Using New Outlook on Windows for professional communication and productivity
{24-04-12 08:29 EST}
Have you migrated your btinternet.com calendar to the Win 10 Calendar "App": Version 16005.14326218540?
If not, then:
My email providers, @outlook, @gmail, and @yahoo, all have a calendar App available for my use if I want to use it. Does your email provider have a calendar App available for your use? If not, then what is your calendar App?
If so, then you need to check Settings for an Export, Sync, or Sharing feature which will allow you to populate Outlook with your calendar entries. I am not familiar with your email provider so I can no longer advise you on this matter. Best of luck. Let the Community know what worked for you.
ps. If you can export your calendar to one of the more popular email providers you should be able to migrate your calendar from its calendar.