Having been a history teacher you likely know that the Royal government issued two emancipation proclamations during the previous insurrection.
Dunmore’s Proclamation, which offered freedom for slaves willing to take up arms against the Patriot rebels; and the Philipsburg Proclamation that more closely resembles Lincoln’s.
Had London won that war slavery in North America would have ended 90 years earlier.
The English did indeed offer freedom to slave who came to their side. But then so too did the Continental Army. But when Cornwallis was under siege at Yorktown, he forced the blacks from his camp to save food for his soldiers.
The English did take freed slaves with them to Canada and later offered to resettle them in Sierra Leone.