As with all exchange, someone will be making a profit, the "post office are one of the "worst" in regard to you getting value for your pound.
Apart from the odd deal with your own bank etc, the best value is in resort, as these local buisness people track the bank rate daily and can give you the value at the time. and make their profit by holding onto the pound and exchangeing when there is a quick peak in the ex-rate, say in mid-winter or whenever.
The only way you can hopefully make a profit is as Steve says try to antisipate the ex-rate and hopefully exchange when it is higher than when you are going on holiday.:- pot luck-

. But with the PO, TC etc they will always be lower than in Icmeler.
Post office is 2.26
ICR in resort is 2.358
Bank rate is 2.368
This says it all !!
So my way of looking at it is "Dont exchange in Uk !!!
Peli