Typical Travelex premium for a foreign currency's buy or sell price over the Interbank rate is 11% for a major currency like the Euro -- and significantly higher for a minor currency. In contrast, the VISA or MC premium to the Interbank rate is 1%. Even a CC that charges a further premium for forex transactions only takes this to around 3%.

The value of UA mile is 2 cents (at most). So if they give you 1 mile for every $2 exchanged, they are rebating you back 1% out the 11+% that Travelex is charging you. So equivalent to using a CC that charges you a 10+% forex fee. Don't do it. Use an ATM.