Translation:
You will come to London. Behold, I warn you, whatever of evil or of perversity (behavior or beliefs that are considered immoral or unnatural) there is in any, whatever in all parts of the world, you will find in that city alone. Do not go to the dances of panders (people who arrange sexual relationships for others), nor mix yourself up with the herds of the stews (brothels or houses of prostitution); avoid the talus (dice games) and the dice, the theatre and the tavern. You will find more braggadocios (boastful, arrogant people) there than in all France, while the number of flatterers (people who praise excessively to gain advantage) is infinite. Stage players (actors), buffoons (clowns or jesters), those that have no hair on their bodies (eunuchs, men who have been castrated, typically to serve in positions of trust around women or in royal households), Garamantes (a North African tribe), pickthanks (flatterers or sycophants), catamites (young boys used for homosexual purposes), effeminate sodomites (men who engage in anal sex), lewd musical girls, druggists (people who manufacture or dispense drugs), lustful persons, fortunetellers, extortioners (people who obtain money through force or threats), nightly strollers (people who wander the streets at night, possibly for criminal purposes), magicians, mimics (people who imitate the speech and mannerisms of others), common beggars, tatterdemalions (raggedly dressed people)—this whole crew has filled every house. So if you do not wish to live with the shameful, you will not dwell in London.
What a crazy context for this excerpt. Clear example of the depth of anti-semitism during this time period.
It is "A Jew" telling a Christian boy where to go in England once he gets there. While the boy thinks the Jew is looking out for him, he is actually sending him to be butchered by the Jews in Winchester, which he characterizes as the most moral place in England after he goes on and on about the wretchedness of all the other places in England the boy might come upon. He gives the boy a note to show to the Jews in Winchester, "written in Hebrew" so the boy doesn't actually know what it says. Boy gets to Winchester and does as told, then soon disappears. Clearly he's been killed by the Jews, but why they had to go to so much trouble to get him to Winchester is unclear.
Oh... Right... That's because this is all made up by Christians desperate for reasons to get rid of Jews.
Holy crap, I took a look at the following passages in the source linked by OP and you weren't kidding.
The text accuses the Jewish community of cannibalism during Passover, with the note apparently being a message to kill and eat the bearer. Gotta keep drumming up that religious hysteria or else people might start seeing Jews as humans.
If there’s one time you know you’re definitely not going to be cannibalized by observant Jews, it’s Passover.
Yes!
Honestly, left out the cannibalism so I didn't sound too crazy.