Here is a graphic that shows that there is a small, very limited amount of interchangeability between metric, AF and Whitworth wrenches (also spanners and sockets).
Of course, it is always best practice to use the correct tool for the job, however, there are occasions where you may not be able to lay your hands on the right wrench size, and you have an equivalent that fits.
It is also possible that a nut or bolt may be heavily worn or damaged, so downsizing a little may give you a more snug, tighter fit on the flats.
Here is a chart that highlights where there is some interchangeability between different types of wrenches/spanners/sockets.
This is also available as a PDF download in case you want to print it out to keep handy in your workshop!
This article is from a series of Handy Workshop Guides I have put together.
These may be of interest to you: