The Crown had no plans to end slavery in 1775. That had nothing to do with the cause of the war. The northern colonies in fact wanted to end slavery at the onset of the revolution. The colonists were upset about having to fight wars for an increasingly distant monarch while they were largely self sufficient, and yet not given power at Parliament. In particular, the French and Indian War fought just a decade prior to the onset of the revolution, was a major precursor: https://en.m.wikipedia.org/wiki/French_and_Indian_War

History disagrees:

- 1706: Slaves entering England are freed

- 1772: no slaves can be removed from England,remaining slaves emanzipated

- 1779: all slaves fleeing from tha US are freed, regardless of their willingness to fight for the queen

So, yes, the crown had legal precendent and plans to abolish slavery as far back as the early 18th century.

Source:

https://en.m.wikipedia.org/wiki/Timeline_of_abolition_of_sla...

English ships hauled slaves around a lot later than that. https://www.slavevoyages.org/voyages/Elnd5Nzy

Those were just the decisions of local of judges which declared that slavery can’t exist in Britain because without additional legislation.

The British Crown and the upper classes had no issue at all with African Anerican and Indian slavery in the colonies and were happy to profit from it.

Abolitionism was a fringe view at the time.

- 1833: Britain forbids slaves in its colonies

Sure, England had a ruling that slavery was illegal in England... but there was no push to interdict slavery elsewhere in the empire (not even Scotland and Ireland, IIRC), especially the colonies. And do recall that some individual Caribbean islands had more slaves than all of British North America.

That's in fact why the British Caribbean didn't revolt during the American Revolution: they were like "what, revolt and risk dealing with a slave revolt without backup from the British Army?"