I heard somewhere that Pitt opposed restoring St. Pierre & Miquelon (and implicitly opposing the fishing rights related to those islands and Newfoundland) to France, in part because he felt that undermining the French North Atlantic fishing fleet would help suppress French seamanship and naval threats in the future.
What if the British government saw it the same way and annexed St. Pierre & Miquelon to Newfoundland and denied fish drying rights there to the French. In effect telling the French if they want Atlantic Cod they've got to buy it from Britain. [Not really, there was cod elsewhere, but this is where a lot of it was].
I see London being harsher towards France on this issue as being more likely than Britain trying to keep Guadalupe for example, because of the British sugar lobby's opposition to added competition.
Would any major British interest have had its "ox gored" by Britain gaining an effective monopoly over the Newfoundland fisheries? Even if it did, did anybody who might lose out by more plentiful fish supplies have a powerful lobby like the sugar one?
And if Britain did this, would Pitt's calculation have proven correct? Would the French have been unable to build up their naval strength as much in the OTL 1760s and 1780s, perhaps leaving them unable to intervene effectively in the American Revolution?