Just to add one thing to what the others have said. If you open an account with GBP as base currency, the first time you trade an instrument outside the base, you get a "sub" account opened in the new currency.
So, for example, if you do some DAX trades you will get a Euro sub account set up which will keep its own separate balance in Euros. You can convert this back to your base currency via the IB FX system, but by default it will remain as Euros.
My account is also GBP base, but I have three separate balances in GBP, USD and Euros. The overall balance is also available and is shown as GBP, with the sub-accounts being converted back to GBP at whatever the prevailing conversion rate is at the time (updated by the second).