Not sure about Barclays, but most UK banks charge a basic fee, plus convert at less than the actual currency rate, so they can hit you with two charges (as I am at HSBC). It depends upon the amount, though. You really need to talk to Barclays themselves.
What I do is build them up ( make sure you check thier date !!!) Im with Lloyds £8 and thier exchange rate , but if your doing a bit at a time the £8 is ok Did open a dollar account but you still have to change it to sterling at sometime Sod the euro if we ever ( which we will not ) loose the £ lets go with the dollar