Actually, it is accurate.

With all due respect, I have an LLB and an LLM so you don't need to explain how things work especially when trying to claim I'm wrong.

Parliament is supreme and the key job of the courts is to enforce the law of Parliament, there is no written or unwritten constitution that gives the courts power to prevent parliament from placing/removing laws as that role is exclusive to Parliament.

Slavery can be made legal if Parliament simply pass legislation making it legal, the Supreme Court could make 100 rulings claiming it's incompatible with English law if they wanted to and not one of them would be binding on Parliament or render them unable to rule on Slavery cases if it was made legal by Parliament.

There was not a "de-facto" ban on slavery since the Norman Conquest, there was simply no legal state of being a slave given it had already morphed into serfdom before that.

Not being explicity legal does not mean it is unable to be legal if there was a law passed making it legal.

In the 1772 case, Somerset v Stewart, Lord Mansfield said that Slavery was incompatible with English law, but that was simply his legal opinion because again, Parliament is Supreme and can do it regardless of what the courts say.

TL;DR - Slavery can be made legal and the Supreme Court can't do anything about it.