I am planning a trip to Berlin, and I have seen that my debit card has no transaction fees or cash fees for foreign currency, so it is charged at VISA's exchange rate. Are there any costs I'm missing, or is this really better than going to a money changer? My account is the Nationwide FlexOne with a Debit card. I found there were no charges here.
To start with, you are right, there shouldn't be any additional fees other than the currency exchange fee - I'm not sure of the exact fee for Natwest, but for Halifax this was around 2.5% for big currencies like the Euro.
However, Germany doesn't actually use debit cards nearly as much as we do here in the UK, so you will almost certainly need cash.
Rather than taking this from a currency exchange booth, what you should do in order to get the lowest fees is head straight to the ATM of any bank, and put your card in to make a cash withdrawal. It will almost certainly ask if you want to use their exchange rate, which it will show you, and you will almost certainly be better turning this down and allowing Natwest to do this for you. Dependent on the bank their currency exchange spread may be as high as 4.5%.
I hope this helps, it certainly saved me a lot of money when I have been going abroad.