The name of _London_ is certainly older than the Romans, and is
probably, therefore, as your correspondent says, British. Its
significance, if any, therefore, is to be sought in Welsh. Now, your
correspondent is certainly quite wrong as to the meaning of _Llan_ in
Welsh. It always means, here at any rate, _church_, not _plain_.
Possibly your correspondent was thinking of _Llano_. The word is written
in Welsh _Llyndon_, or _Llyndain_, which also speaks against its being
compounded with _Llan_. The word certainly _might_ mean anything: but I
know of no satisfactory explanation having been given for it as yet. The
only words for _town_ in Welsh are, I believe, _tre_ "city," or _caer_
"castle,"--as parts of compound words, I mean.