The answer to your question is that there will not be a nasty surprise. You will lose only 0.3% on the P/L transfer to SEK at the END of the trade. So that is only a tiny bite out of the exact profit or loss, not on the underlying. You are likely already paying this on your demo account, but it was too small to detect.
The exact details depend upon whether you have a CFD or spread betting account. In your case (I am guessing) of a euro-denominated spread bet on EUR/USD making EUR 100 profit/loss, you will receive EUR 99.70 / pay EUR 100.30 worth of SEK. It doesn't matter that the underlying may be EUR 100,000, that doesn't enter into the calculation.
Your demo account is identical to a live account intra-day. (But it doesn't reflect overnight finance charges, where the 0.3% currency conversion charge to SEK, though still there, will be lost in the rounding error.)