Is there a way to set the default background colour of each new layer to white? Currently each layer I create has a transparent background.
Create an Action that will make a new layer and fill it with white, assign it to a hotkey and use it instead of default New Layer command.
New Layers are always transparent. there's no method to have all new layers be white. You simply need to fill the new layer if you want it to be a color. With default ... you could .... New Layer then Command/Ctrl + Delete to fill with the background color (white by default)
You could use
Layer > New Fill Layer > Solid Color to add new layers, but this may not be what you are seeking either. It creates a color layer with a mask, not a simple layer with a color fill.