My favorite place for soundproofing knowledge and products is SoundproofingCompany.com some great articles there.
What is a shire? and it would help us to know where in the world you live so we would have insight into the building materials available.
Roof, I was thinking replacing the metal with roof made with wood trusses, plywood sheathing, tar paper and shingles. However if the metal is sound it might be easier to spray an insulation/sound deadening coating on the underside, then building your recording studio as a free standing building inside the existing building. One concern is the ceiling height, looking at the video.
A freestanding structure would have four walls a few inches from the outside wall, built on top of a new floor that would be floated on rubber pads. You would rest ceiling joists (cross members) on top of the walls spanning the ceiling and not touching the existing structure. You would add 2-3 layers of 5/8 drywall with Green Glue to the new structure (Inside only) and insulation.
Knocking down the structure and hiring an architect to draw your plans for a free standing recording studio is a very expensive pathway. Probably $50-75K. I think you would want a simpler DIY path. I think you ought to start down your path making do with what you've got with some modifications. Something you can do yourself.
It is before your time but one of the original rock bands was Buddy Holly and the Crickets. They just hung some cardboard egg crates in a garage to tame the echos and made their first recordings there. When they listened to the recordings they could hear crickets in the garage and hence the name.