When setting a pending order (Stop, Limit or Stop-Limit) on the Invest or ISA, you can see at the bottom of the order’s tab that you have two options for setting an Expiration - End of Day and Never.
With the 'End of Day' option, the pending order will be automatically cancelled if it's not executed by the end of the trading day on which it is placed.
In contrast, by choosing ‘Never’, your orders will remain pending until they are either executed or cancelled from your side.