Because Mexico learned English from Britain, he uses a lot of the terms that are not common or mean the same in the US like chips = fries, rubber = eraser, biscuits = cookies, etc. Mexico also uses a lot of British slang like twit, chap, wanker, bloody hell, bullocks, blimey, rubbish, grockel, etc.
Britain usually gets cocky and a huge ego boost that Mexico uses his terms instead of America's. On the other hand, America is rather pissed that Mexico doesn't speak his English and teaches him the "correct" way of saying a certain word.
Mexico gets confused about this as he thinks all English is the same and that there is no different dialect for English-speaking countries. Nonetheless, the other nations think it's endearing that he imitates Britain's way of speaking.
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Example:
The meeting continued on with Germany yelling at everyone like 99.9% of the meetings. Mexico dozing off on the table half-heartedly listened to the German's words not fully basking the meanings. He watched the other nations chime in here and there when it came to finding solutions to some of the world problems discussed in this month's meeting.
Soon, the infamous North American country, America chimed in with probably another weird scheme. "How about we put a bunch of towels by the shore to absorb all the water to prevent rising sea levels!?"
Silence engulfed the room. Mexico lifted his head up from his palm that was previously perched on top. Blinking his brown eyes with curiosity and cocking his eyebrows in astonishment he spoke.
"You lost your bloody mind! Your rubbish solution is doomed to fail! What kind of bloke do you have to be to be this dumb?!" Mexico exclaimed furiously in English, his accent lacing around his words like silk.
America glared at his southern neighbor. "Speak normally!"
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Needless to say, he hates Mexico's British style of English and Britain loves it.