I'm going on this tour in March.
Seeing as the Myanmar Government is trying to stop USD being accepted for any spending purpose in an attempt to strengthen their own currency (complete opposite to Vietnam's and Cambodia's approach who want tourists spending strong currency to boost their foreign exchange reserves), I'm just planning to take out 300,000 kyats (about £162) from an ATM in Yangon Airport on the way in - of which there are now many according to many forums. 300,000 is the maximum you can take out in a single transaction and there is a local bank charge of 5,000 kyats per transaction in all ATMs - having read that on other forums too.
If I have anything left it at the end of the trip it will either go on duty free on the way out, or changed back into GBP at the airport.
I'm not taking USD to then change them into MMK as you're effectively paying two sets of commission from GBP. Also, I bank with HSBC and have an account that does not charge extra fees to take foreign currency out of my current account via an ATM, just their rate for the exchange which will always beat a cash conversion rate from a non-convertible (non-tradeable) currency.