The Bible does not forbid slavery. But the Old Testament regulated it. Paul encouraged masters to treat their slaves fairly and for slaves to obey their master. Slavery was not tied up with race and skin color in Paul's day or Abrahams like it was in the US South in the 1800s and prior centuries.
Abraham's servant, the one whose prayer the LORD answered, said of his master in Genesis 24:35,
And the Lord hath blessed my master greatly; and he is become great: and he hath given him flocks, and herds, and silver, and gold, and menservants, and maidservants, and camels, and asses.
America is not Egypt and does not resemble it much at all. Americans named some of their cities after Egyptian cities. They don't look like Egyptian cities. The Nile is big. The Mississippi river is big. The religion and demographics are not the same. And 400 years is not 245 years.
The British Israelite movement found vague similarities between passages of scripture and the British people. That doesn't make the British Israelites.