Tesco currency for me.
Where to buy travel money at a good rate?
got a quote from them for a family holiday, they want 4x the amount for the insurance (based on other quotes) and they give a crap rates, i'll pass
If its not to late get a Halifax Clarity card ,pull out cash from an ATM when you get abroad at the real interbank rate (better by at least 5% than what you will get at the Post Office ). Other cards /methods are available but I suspect that the method you suggest will not get you the best overall deal . :(
I would hazard a guess that their Travel insurance rates are not the best either . Check on a compare site - As you always should .