Different debit cards have different rates applied to them (and different card machines might make different charges).
As a rule of thumb, most foreign currency outlets (such as the Post Office, M&S, Asda, etc) look at the international exchange rate (that banks apply when converting currencies) and then charge you very roughly 5% more to get Euros and give you about 5% less for changing them back to Sterling.
Using overseas cash machines often gets you the international exchange rate (which initially makes you better off) but then you pay a fee to your card issuer and/or the bank owning the cash machine. Sometimes that's a percentage but it might be a flat fee (or a percentage with a minimum and/or maximum applied to it), so making direct comparisons is extremely difficult.
I've found taking out cash abroad (using my Barclays debit card) seems to be best for me but to get the best deals it's always wisest to see if your bank has branches (or 'partner' banks) in the country which you're visiting. If you use those banks you'll either avoid any fees or pay lower fees.
However, to get enough Euros to 'get me started' abroad, I use either Asda or the Post Office.