I believe that Avon in England is not Anglo-Saxon except by adoption - like Derwent/Darent, Dour, Dee and so on, river names tend to be Celtic, even in the east. You could call words like pyjamas, bungalow, and didgeridoo Modern English, justifiably, but it does rather obscure something essential.

There are a couple of basic rules you need to remember - what is the earliest the name is known? If it predates the supposed reason for the name, then it indicates something about the folk etymology. Do the earliest versions resemble other words than the ones they resemble now, which may have been influenced by folk etymology? And places are more often named for features of the place than oddities of people, so you need to know the geography.

Penny