Google maps gives you a good idea for City a to City b. If you are planning pallet network multi-drop in London its going to fall down but city to city its pretty reliable.
It would mean a bit more work, but I would run Google which is free, against tom-tom for a month or two and see how many times google gives a drastically shorter route by going through weight limits or low bridges. There is no point in looking into more depth into expensive truck planning software if Google can do the job. I would have done it my self but I don’t have a Tom-Tom planner to compare it against.