With any other broker it'll be massive, but with IB it's minimal, because you can execute the two trades at interbank rates. Say the spread is two ticks on GBP and five ticks on SGD, that works out to about a twelve-tick full spread on GBPSGD - or very roughly SGD 6 per SGD 10,000 if you combine the buy and sell conversions. Pretty cheap.
This is why you make a will and leave the account details with your lawyer (who presumably will not be a doddering 70-year-old). Leave an instruction that says "here is the username and password; here is a copy of the code card; in the event of my untimely demise, immediately close all my open positions, withdraw the cash, and spend it on hookers and blow". Or "in the event of my untimely demise, just, y'know, the account's yours, do what you want with it, here's how you log in".
You'll have this same issue with any other online broker; it's not unique to IBKR.