I don't think a Commonweath trading area is a fable. Certainly not such areas as Canada, Australia, New Zealand, and the West Indies and Pacific territories.
India, Pakistan, and Africa would be a separate matter....
While not of a size to be a permanent trading block in and of itself, it could form the basis to negotiate free trade with the US on a somewhat equal basis and then from there arrange free trade with other first world economies such as Japan, South Korea, Europe, etc...