eg. If I bought the steam wallet code in the UK, would I be able to activate it while on holiday in Hong Kong?
A Steam Wallet code purchased in any country can be used in any country's store.
However, if your wallet code provides value in a different currency, you will be prompted to convert the funds into your store's respective currency.
For example, a SGD $10.00 code purchased in Singapore will yield approximately USD $7.30 on the Australian Store.
The example below shows you a conversion between a code purchased in Europe used in the US store.
Unlike iTunes, a code can be redeemed anywhere on any store with any currency.
Also, you do not need to be in said country to use its store. As long as your store region is set to your home region, you can purchase, redeem codes (to be converted to its currency) at its prices.
Disclaimer: The amounts shown in this answer are estimates at the time of writing. Please check with your local currency's up-to-date exchange rates.
Also note that the conversion rate Steam uses is lower (less bang per buck) than the official rate.