Take a tip: look at the map or use viamichelin to find the main routes, then use the satnav as a glorified street map. That way you will undoubtedly get the best route.
The problem with satnavs is that you only get the option of shortest or quickest route - the latter will insist on motorways even if it is out of your way, the former will draw a line from A to B and go on the nearest road/lane/track to it. Ideally you want a 'main routes' option which will do the A-B option but then try to get you there by a combination of motorway and main roads only going on to minor roads when there is no alternative - but that seems to be too easy for the manufacturers to grasp. On my TT I set it to limited speed (50mph) and it seems to give a better solution.
One thing that it took me a long time to get my head around is diversions. I live in Harrogate and used to go to St Helens occasionally. TT would tell me to go through Bradford, then M62 and M57/A57. My preferred route was A59 to Skipton, A56 to Colne, then M65/M6/M58/M57 - longer much much less traffic and far fewer holdups. If TT had decided to go the M62 route it would do everything it could to get me back onto the M62, the last attempt being the A556 between Burnley and Blackburn. Only when I got on the M65 would it decide there may be a better route. However stop near Skipton, clear the original route and ask it to calculate from there and it would take my preferred route immediately.
The one thing a satnav is good for, wherever you are, is to tell you how far it is and how long it will take to your destination - and it is usually right within a few minutes whichever way you go!