Oanda's example is based on a USD denominated account. In order to determine USD position value and USD margin requirements you need to convert all positions to USD. That's what the EUR/USD example did.
If you are trading GBP in a GBP demominated account, then no conversion is necessary. If you are trading 10,000 GBP/JPY, then your position value is 10,000 GBP and your margin requirement is 2% x 10,000 = 200GBP.
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