Currency updates rely on you setting one currency as the DEFAULT. So if you are working with BRITISH POUNDS, this is the default and the value is set to 1.0000 (Use four zeroes for rounding purposes).
Then, in your currencies admin, UPDATE them (I usually do it daily). This gets the latest exchange rates from Oanda and / or Xe and they are applied relative to your default currency.
You can also apply a bankcharge offset (can't remember where the switch is - thinks its under Configuration > Shop, so you recover that bit your bank creams off the top. (I'D BETTER READ THE NEW BOOK AGAIN - AS SHOULD EVERY ZENNER!!!)
There's a mod out that will help you set auto-timed updates using CRON on your server, but unless you're familiar with this, don't try it just yet.