Why did the Brits get so many place names wrong during the time of the Empire?

Because this ridiculous habit has always been a tendency of the British. Being an island race and having the comfort of being surrounded by an ocean, we have always been slightly isolationist. We like it that way. The idea of some foreign power living on your doorstep is just slightly obscene. It is also the reason why by comparison with Europe we are so poor ((and reluctant) to become very proficient at foreign languages.

We counteract this lack of skill by giving English nickname to foreign cities and places only remotely similar to their actual names. You see, we believe that we got it right, its the foreigners who somehow got it wrong, , but after all, as lots of them cannot speak English, what do you expect ?

They didn't , you just don't understand English.

They didn't and your hating the British ISN'T going to change history or the Facts.

You are such plebe.. the names of places where Anglicized by the the Empire...

How did the French get so many place names wrong? Like calling Deutschland, Allemagne? Or calling England Angleterre and America L’Amérique?

It's not necessarily "wrong", it's that the local populations themselves speak different dialects or completely different languages. The official name of a place may change depending on which group is in charge.

Why do you use 'Cymry' (Plural),instead of 'Cymro' (Singular).

They didn't 'get them wrong'. They just created a version of the local name that would come natural to English-speakers, just as they had been doing for a millennium with foreign place names and as they still do with European ones: e.g. Rome, Moscow, Spain etc, instead of Roma, Moskva, Espana. Also, in some cases they use a different name entirely, such as Germany instead of Deutschland.

Other European nations had the same custom: the French say Londres not London, and the Germans say Rom not Rome. It's a very recent fashion to try to pronounce the names of far-off places the way the locals do.

They didn't

The former are phonetic representations of the city names in the latin alphabet

and

the latter are phonetic representations of the city names in the latin alphabet.

Madras was an extraordinary case.

Maybe ye confuse brits with empire.

At the end of the world according to revelation is a dragon.