Officer on duty to explain the meme!
The pound is a unit of "mass" used in the United States and other systems of measurement. So if someone obese loses 50 pounds, it will make him happy and that's why John Cena is smiling. However the 2nd part says that you're British and John Cena is angry/shocked. This is because "Pound" is the official currency of the United Kingdom and 50 pounds equal to 65$ approx. So he actually lost a lot of money as he's British and "pound" is a currency not a measure of mass as in the US.
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Difficulty level of the meme: 6/10