I can't comment on the specific journey you are doing, but one I do quite regularly, a route I know very well, my Satnav puts in various bonkers detours - I guess it's basing on the most direct route or whatever preferences I have selected - but it always seems to be way over the time expected and suggests various roads that are clearly not the best way.
My experience of Satnav generally, if you plan to go somewhere, always know roughly how to get there and then use Satnav to help you on your way. Sounds like you are doing the right thing in checking your route before you go!
Not exactly much help, but you are not alone!
The Saint.