I have just driven from the West Country to East Anglia and have experienced exactly as described above. Deviation from roads through narrow country roads only to re-emerge back onto the same A road. I found it easier to switch off and ask my wife who was a passenger to guide me using a map. It rather defeats the object of having the sat nav. Tom Tom really must resolve this.
This of course begs the question, if it's possible to select motorways to be avoided, why can't unclassified roads be selected? How can this be "related to map data" and be "rather complicated issues to investigate" or is the map data that c**p that it's not possible to know whether a road is an A/B road or not? Either way, this is totally unsatisfactory for any satellite navigation device, let alone these expensive units, which not only can't plan a sensible route but also can't hold a battery charge for more than half an hour or so if you're lucky. Perhaps we should start lobbying the elected members of the government to consider having these devices banned until they are fit for purpose as they are potentially putting people's lives at risk, even in the daytime, let alone at night. As for my GO 5100, it can stay in its box in the loft, ready to go on e-bay or maybe the bin as I'm not entirely comfortable asking someone to part with their hard-earned money for this overpriced piece of [add your own expletive to suit].