On weekdays (Mon-Fri London time) the interbank exchange rate is applied to all transactions with exception to Thai Baht and Ukrainian Hryvnia where there's a 1% markup on top of the interbank rate.
If you exchange currencies on weekdays (Mon-Fri UTC) you will receive the interbank exchange rate without any markup on all currencies except for Thai Baht and Ukrainian Hryvnia where there's a 1% markup, with a small 0.5% fee for anything above S$9,000 per month.
On the weekend (Sat-Sun London time) the provided rates are fixed to protect against fluctuations, this means there are markups of:
Also, make sure that you always choose to withdraw cash in the local currency when you are abroad. Some ATMs will let you withdraw cash in your native currency but they will apply foreign exchange fees for doing so.