To me it is a piece of a cake ( or it became a piece of a cake ( I learned even though I am a native speaker....)).

>(2) イギリス人から軽んじられた。

「から」,which is used when in the Japanese passive voice **something is moved** ***by the de facto subject.***

So here, (2), the actual ( de facto ) subject is イギリス人。

What was moved by the イギリス人？ I think it is the **feeling of "disregarding", "looking down" etc ( which the English man/woman has inside his/her heart )**. The feeling which the English man/woman has was **moved** to the person ( here unknown ) so that the person can feel he or she is looked down.

Whereas, 虎から食べられた。'case, 

**Is there anything that the subject 虎 can move or shift to the person**?

No, the motion, **To eat** can not be **moved.** Upon being eaten, the person ( the object ) dies. So nothing moves. So the latter can take only 「に」here, which denotes simply the actual subject, 虎．　

If we talk by the active voice, that might be easier to understand.

**The English man/woman looked down on him/her** --> The feeling ( which has the English man/woman has ) was moved to the person so that the person can feel he/she is looked down by the English man/woman.

**The tiger ate the person** --> "To eat" can not be moved because instantly when the tiger eats the person dies......