Full details of the T&C of the Upper / Premium pre order facility can be found here -https://flywith.virginatlantic.com/g...re-select.html

I have only skim read the detailed T&C, but I can't see anywhere obvious that booking via Delta would disbar you.

Having said that - I don't know, in this day and age if Delta can share your email address without your consent, so it may be something you have to be proactive in.

Nick