There's some discussion here but I'm not sure if it will help:
https://community.bt.com/t5/Email/you-have-moved-this-message-while-using-another-device-at-the/td-p/2084166
Webmail has always been second best to accessing email accounts through a dedicated email client. (When email was invented, there was never any intention that it could be accessed via web browser. There was an expectation that people would use Microsoft Outlook, Outlook Express or something similar in order to use it. So webmail is trying to use something that's not really 'right' for the job).
My own instinct in this situation would be to install a proper email client, such as Thunderbird
https://www.thunderbird.net/en-GB/
and configure it to work with your BT account. You should then (hopefully) be able to remove or delete the troublesome emails. (From then on you can choose either to use webmail or Thunderbird to access you account. They'll both work).