If things are as lax and undocumented as you suggest, wouldn’t a better approach here be speaking to your employer/line manager/budget owner to ask them: are they happy for you to book economy, submit the expense, then call BA within 24 hours to change to business? If there is no corporate TA involved it should be pretty straightforward, and having even an informal blessing on this keeps you covered. For simple point-to-point Club Europe tickets, you get the added benefit of same day changes (assuming your employer is insisting on Basic Economy).
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