Not the Netherlands (or Sweden I believe). The Netherlands had not been involved in the seven year war and was becomingmore and more neutral. I doubt it would support France in a war over Lorraine. It is more likely they remain outside the war entirely, just like they did in the seven year's war.This would be a serious conflict with Britain-Hannover, Russia, and Prussia against Austria, France, Ottomans, Spain and likely the Netherlands and even Sweden.
The primary objective for Britain would be that Flanders would not end up French. They would give it to the Netherlands, if they don't want Austria or Spain to have it, but they would be content for Austria to keep it. As the Netherlands would not get involved in the war (assuming France doesn't attack the Netherlands), they would not gain part of it. It is also doubtfull the Dutch would actualy want Belgium.Whatever territory the British gain in America, it seems unlikely that Flanders will wind up in the hands of a friendly power. The current owner, Austria, and the two neighbors (Dutch/French) who can lay claim to it through ethnic and linguistic ties are hostile. Britain would seem unlikely to have it ceded to Prussia, or annexed by Hanover. So who gets it assuming a British win? A hostile power holding the ports of Flanders is a direct threat to Britain.
Flanders BTW is not for Britain to give away. Especialy as I doubt Britain (or any of the British allies) would be able to occupy it.