There are many ways a currency can lose its value. For example, during the financial crisis people were dumping GBP in favor of reserve currencies such as USD, which will probably happen again during the next recession, whenever it is.
Taking the last recession as the example, if I were to sell 100,000 GBP in mid-2008 for 200,000 USD, then by the year end I'd still have 200,000 USD, which would be worth 133,333 GBP. I think I preserved my capital. Does the revenue service think I made money?
Update: asking as a resident of UK, but would be interested to learn how it works in the US as well