I suspect (with nothing to go by) TT wanted to wait till the rush of people updating to 885 maps was over before implementing the ability to buy new maps of other regions and be able to move them to the sd card if there is no space on internal.
Meanwhile, you can test the slot to see if it's working -- if you can find a uSD card that doesn't have anything important on it, blank format it to FAT32 with Windows and install it into your unit; you should see your 825 suggest formatting it to the TomTom spec.
When the time comes for buying maps, you'll likely want to consider a 4GB Class 6 card. Since you are in Europe, I am guessing you already have the unit with 4GB of internal memory (serial number starts with FW), not the 2GB unit (serial number starts with FY). The latter only allows for 'skinny' pieces of Europe to be loaded now.
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