The believability of the article at your posted link is utterly invalidated by this single statement. I'll ignore its many and varied other idiocies.
No such document has ever been found, or ever will be found, since it never existed. It has been invented out of thin air by idiots such as the writer of this article.
The Mahdi Army was finally defeated in 1899. At that point the British army, navy and government had been continuously campaigning against slave trading for over 90 years. It was one of the few consistent policies followed by all British governments over this entire period. BTW, the US efforts against the slave trade were minimal, and often slavers hid behind the US flag to protect themselves against British search. Not our finest hour.
AAMOF, the Mahdist revolt itself in the Sudan was largely caused by British efforts to stamp out the slave trade there.
The idea that any Brit of the time, much less the Queen (!) would give anyone a license to continue slave trading is beyond parody.
potted history of the period:
http://www.pbase.com/perrona/image/86057566
and nothing is ever quite as black and white as you might wish it to be.
I will quote from the book The White Nile and allow you to read between the lines.
‘Gladstone said that he was prepared to take a chance with Zobeir, even though he was bound to face a vote of no confidence in the House. Queen Victoria was consulted and approved. But the other members of the Cabinet were much alarmed; already the public had been asked to swallow Gordon’s proclamation legalizing slavery in the Sudan and now it was altogether too much to expect them to accept the employment of “the greatest slave-hunter who ever existed” ...up to this point the correspondence about Zobeir had been kept secret....