Yes,you can - but you can also use ATM,s ashore which will probably give you a slightly better exchange rate.
You will probably get a better rate and avoid commission if you buy before you go. personally I use either the Post Office ot M&S.
This is a difficult one because one doesn't know how much one will need. I can only speak for Norway where we went to Flaam, Stavanger and Oslo and we used euros the whole time with no problem. Just for coffees, newspapers and small souvenirs. Most things were very expensive so we didn't want to spend any more than that...but we have a nationwide (which doesn't charge very much, if any, commission Credit card which we would have used in the ATM if we wanted/needed any local currency. If you get the money before you go the chances are you will over estimate your requirements and so lose out when you change it back to sterling!!