You can choose – there are over 300 highly-rated international investment funds from a range of major fund managers available for you to select. You can choose up to 20 different funds to hold at any one time, decide on the percentage allocation to each, and make changes whenever you like.
By default (at the start), your savings money will be 100% invested into the iShares S&P500 ETF, which tracks the S&P500 Index.