I wouldn't call it a Bri'ish W. It was opportunism. They foresaw the loss of their colonies and so decided to use a bunch of black men as human shields in a desperate attempt to keep those colonies. It was less driven by an ideological sympathy for abolitionism than it was driven by a desire to keep their colonial holdings. The British also opportunistically supported the confederacy during the civil war because they benefited tremendously from the American cotton market in their textile industry. Had the British held onto colonial America, it is very possible they would have betrayed the Ethiopian regiment at the end of the war and put them back in chains, or even more likely, just keep using them as cannon fodder until they all died.