In this case, "Watashi wa Igirisu ni iru." is correct. "desu": "A is B." means "A = B". So, it is translated into "A wa B desu." If above sentence is "I am England.", it is translated into "Watashi ha Igirisu desu." (of course strange.) "iru": "iru" is near meaning of "stay" or "exist".

yes, the correct one is watashi wa igirisu ni iru 私はイギリスにいる。 or watashi wa igirisu ni imasu.　私はイギリスにいます。 as Takashi says iru / いる means exist. if you want to say "I'm England" that would be watashi wa igirisu desu.

Watashi wa Igirisu ni iru...RIGHT Watashi wa Igirisu ni desu...WRONG.

Don't trust the translation too much between Japanese and English/French whatever. Our grammar is so different that there's some words we don't have in common such as "desu"! Wwatashi wa igirisu ni iru is correct!