Yes, there's plenty to back it up.
Should be easy for you to cite one authoritative source which states in general rebuilding old structures is not worth it compared to building newer and more efficient buildings. Should be a sinch!
owner would redevelop this site to 'consolidate' in a prime central London location
You have no idea what you are talking about with regards to this development and probably property either.
Your arguments will be more compelling if you aren't making it up as you go along.
They are redeveloping the site and consolidating because M&S don't need 5 storeys of retail space. They need 2 and want to turn the rest of the footprint into purpose built offices for rent.