There are no issues with updating a device that has an available uSD card. However, there are two different models of the 825 (2GB and 4GB internal memory) and it was my impression that neither had an external uSD slot. That he found one on his unit is good news. I may have to update my chart. If the OP had the 4GB model (serial starting with FW) loading up a Western Europe map or adding France would be a piece of cake. However, based upon his post, I suspect he has the 2GB model (serial starting with FY) and would possibly not have enough room to add even France to the unit without adding a memory card.
That said, the MyDrive Connect software is stable, and it's not at all difficult to swap maps on and off units these days. Unlike the 'old days', map ownership data are held 'on account' on the TT server by device serial number, so you don't even have to worry about backing anything up these days.
@kontiki bill
As dhn suggests, you will need to use the configuration option to switch maps. Unless/until you're there, it will keep you located in the UK. Once you've switched to your France map in the configuration, you'll be able to search and review the map on the other side of the channel.