I don't disagree with you about overall forest loss, but the great oaks were the first choice of shipbuilders. Long-term planning wasn't a government strength then, so perhaps little has changed.
...but even then, the amount of mature, tall timber available for single-tree masts, was in rapid decline. Merchant and naval ships alike were being built at a tremendous rate (often as replacements, because thousands of ships were lost at sea), and England shifted from being an exporter of this kind of wood to a net importer. Imports from the Baltic met some of the demand, but as ships grew larger, the necessity of making masts from several shaped long timbers advanced the technology. This was the interpretation of several maritime historians in quite a number of books I've read. Of course, modern research may well have shown that things were not that simple...
I still find the argument unconvincing. The point is that, even by the time of the Norman conquest (and probably long before that), woodland in England had been reduced to a small fraction of the country's land area and was being pretty intensively managed. Rather than ship building having 'used up' all the big trees, the reality is much more that the woodlands were being managed much more for coppice wood products because that's where the demand/money was.
Regarding masts, I don't think large open grown trees are that great for these anyway since they tend to be rather crooked - fine for curved ship's timbers, not so great for masts. Tall, straight conifers from the Baltic would be much more suitable anyway, and given the difficulties of overland transport would in all probability be cheaper to import by sea anyway. Increasing use of imported wood can be just as easily explained by it being cheaper and more suitable for large ships, rather than implying that Britain had 'run out'.