re: I have heard that "The Patriot" is terribly inaccurate and offensive to the British

The British were also pissed that the film wasn't honest about America itself and not just British bashing.

I read a review in the Guardian that complains Gibson shows the Americans as can do no wrong moral beacons and the British as colonial slave-holders.

According to The Patriot, slavery was practically nonexistent in South Carolina and really not that bad, anyway. The few slaves shown are a cheerful lot, all of whom have been given their freedom to retire to a beachside cabaña. There's even a token slave in Martin's militia. "We will have a chance to make a new world," Martin's son Gabriel (Heath Ledger) tells him earnestly, "where all men are equal in the sight of God." "Equal," intones the slave. "That sounds good."

Don't get your hopes up, old chap. It took the civil war to end slavery in the US, almost a century after The Patriot is set. Even then, South Carolina was on the wrong side, being so attached to slaveholding that it was the first state to secede from the Union after Abraham Lincoln's election.