If your available balance is in a different currency (e.g., SGD or USD) from the LSE security's trading currency (e.g., GBP or EUR), Syfe automatically converts the required amount at the prevailing FX rate at the time of execution. No separate FX order is needed. If you cancel the order, no FX conversion is performed and your balance remains unchanged.

In which currency will my LSE holdings and P&L be displayed?  

Each LSE security is held and reported in its trading currency (USD, GBP, or EUR), while your overall portfolio value may also be converted into your base currency for consolidated reporting.