I can expand the canvas to the right or downwards without a problem, but is it possible to expand it to the left or upwards? Of course I can just move the whole drawing to the right, but the problem is that guides will stay where they are.
Here's a simple method
Make sure Snapping is enabled, and Snap to Page Border is also enabled, then draw a rectangle to fit the page size. Snapping will help you get it exact.
Change to the Select Objects tool S, and scale the rectangle to the left by clicking and dragging on the centre left handle. Note: If you need an exact size you can enter the required width/height manually in the options along the top and then make sure you reposition it by snapping to the existing page.
With the rectangle still selected, hit CTRL+Shift+R which will change the size of the page to the selection.
Finally delete the rectangle