Scenario 1: A specific person claims they were the victim of a crime, and give specific details about it.
Scenario 2: Someone on the internet says that 18 crimes occurred last week, but wont give any details or information about them.
If you genuinely can't see the difference, try to picture the criminal trial for scenario 2. There are no specified victims or defendants, just some person saying "some crimes occurred"...
As to specific instances of rape that have been mentioned (the Guardian link you posted, plus Harrow and Canary Wharf).
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in the Guardian-reported cases, the victims claim to have been raped by asylum seekers.
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in the Harrow case, a man claims to have been raped by another man. All that is publicly known about the accused is their name (Bruke Desalagne), age, and that they had no fixed address. People on the internet are claiming (based, seemingly, on the surname being of African origin) that the accused was an illegal immigrant.
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in the Canary Wharf case, there is no public information about the accused (or even if the victim saw them) and the alleged rapist has not been identified. Some people on the internet as claiming that the rapist must be an illegal immigrant, "because".
I believe all of these incidents very likely occurred. I believe that the ones in the Guardian link involved asylum seekers, because the victims have said as much. In the Harrow and Canary Wharf cases, I neither believe nor disbelieve anything about the immigration status of the accused, because the victims haven't alleged anything about that.
I can see why some people might leap to conclusions based on the surname and homelessness of the accused in the Harrow case - and they could be proven correct. I don't know why anyone, capable of critical thought, would believe the (so far, unidentified) accused in the Canary Wharf incident is an illegal immigrant just because someone on the internet says so.