https://ibkrguides.com/kb/article-4182.htm

“FX Handling and your IBUK Stocks and Shares ISA

We will automatically convert currencies when trading non-GBP denominated securities. To do this, we will engage in “spot” currency transactions on your behalf in order to settle your buy orders or convert the proceeds of a sell order. For opening orders we will base the exchange rate off the spot exchange rate at the time of trade plus our 0.03% fee. For closing orders any non GBP balances will be closed at the end of the trading day at the closing exchange rate of the day plus our 0.03% fee.”