in several colonial legislative bodies to slow and even ban the importation of slaves since the mid 1700's. Even in Virginia and Maryland. However, London, and their appointed colonial governors would have none of it. It was far too profitable for the empire and the crown. But, remove the North American leg of colonial trade (fall out from our revolution), and slave importation became less lucrative on the whole for Westminster. Add to that a growing moral indignation in the UK about slavery in general and you have the Act you mentioned.
Much of that was reflected in the American colonies, thus the attempts to slow or ban slave importation into New York, New Jersey, Maryland and Virginia to name a few. Only the far southern colonies of South Carolina, Georgia and to a degree, North Carolina weren't on board. Some of the reasons for a ban were economic, but many were based on moral grounds. And not just Jefferson. It's a theme noted throughout the writings and speeches of the Virginia radicals, but also some centrist and conservative Burgesses from the Tidewater. Simply put, the importation and keeping of slaves was completely incompatible with the Enlightenment thinking these men subscribed to. A moral, economic and philosophical conundrum they struggled with all of their lives. Easy for me to pass judgement from this perspective, but I wonder how we would have reacted if we'd been in their shoes?
https://allthingsliberty.com/2020/09/the-first-efforts-to-limit-the-african-slave-trade-arise-in-the-american-revolution-part-2-of-3-the-middle-and-southern-colonies/
The Brits did ban slavery outright in 1833 and didn't need a bloody war to do it. I'll certainly grant them that.