There are alternatives, like investing in ETCs, e.g. Long USD/Short GBP; Long EUR/Short GBP; etc.; or vice-versa.
In reality when we buy a foreign currency, we are buying it (going Long) by selling another currency (going Short).
(The same principle, goes when we buy stocks/other instruments in foreign currencies, we are going Long in the instruments and their currency and going Short in the currency we sell. For example, buying the original USD-denominated Apple stock with GBP or EUR, etc, we are going Long on the Apple stock and in USD, and we are going Short in currency used, the GBP or EUR, etc)