Delta puts restrictions on Virgin sizing
Delta Strikes Pilot Deal to Keep Jobs From Outsourcing to Virgin - Bloomberg
Delta can negotiate what they wish with their pilot groups, but it seems to me that this restriction of the airline of one country on an airline of the other country is not permissible under the Open Skies agreement.
"The union sought to protect the bulk of Delta’s U.K.-U.S. flying and ensure that Virgin doesn’t erode operations on other routes around the world."
No, Open Skies means you can't do that. Let's see if the CAA can be bothered to do anything about it though