Trying to better understand basic grading levels.

[Richym99]

Clearly, even to this hewcomer, grade of a coin is as important or in some cases even more so than it's age or rarity. I've read the guidelines on how to assess the various levels of grading, but it still difficult to visualise how those guudelines translate, giving often very small distinctions between the grades. 

These are three of the sovereigns that I have acquired since I started collecting in March. Would anyone care to offer some guidance on what I should be looking for and what they see which affects the grading of each up or down.

One I bought off a member, the other two were part of a collection sold by a dealer.  How would you grade the Obv and Rev of each of them? 

For clarity these are an 1898, 1900 London and a 1920 Perth. 

Thanks for looking.