Not pre-ordered any games via the eShop myself, but from Nintendo Pre-order FAQ page :
What happens when a game is pre-loaded?
A non-playable version of the game will be pre-loaded to your system at the time of your order. You'll see an icon for the game on your HOME Menu, but you will not be able to launch the game until it is officially released. You'll be able to start playing after downloading a small update on the game's release date. (Please note that some third-party titles are not available until midnight CET on the game’s release date.)
How do I get the software update that unlocks the game on the release date?
As long as your Nintendo Switch console, Wii U console or Nintendo 3DS family system is connected to the internet, you will be prompted to download the update data once you launch the game. Additionally, if you have your console in standby or sleep mode and connected to the internet, the update data will download and install automatically.
So looks like you download a small update to unlock the game and they are not providing any specific time, but I thought the eShop updates at 2pm BST.