NanaTuesday, I don't think you're being petty, as this tradition over the past years has been a big part of your life, and something you've enjoyed and looked forward to. I can totally understand that you no doubt feel upset that it's come to an end, and particularly the fact that you feel you've been excluded from the family plans of a holiday abroad.
However, perhaps it's just their way of trying to say that they want something different now, without actually having to spell it out to you. It could be that now your GC are all getting older, they want something different, and they may have told this to their parents ... kids are not slow in coming forwards, and they would have no real idea that it would probably upset you so much. They may no longer want to go on a road trip around the UK with their grandparents, but instead want the excitement of a holiday abroad. Even close family can feel awkward/embarrassed about having to tell someone face to face that they want to change things.
I do understand the fact that your ex husband has been invited to join them makes you feel excluded. However, as you've said yourself, "he is the parent who has nothing else to do, and you are the parent who has a life outside of DC and GC." If he has no partner, and he doesn't have close friends that he can spend time/holidays with (particularly if they're mostly married couples), they possibly feel sorry for him, and wouldn't want to leave him alone. He's no doubt lonely. Whereas, you have a husband to spend your life with, to do enjoyable things together, to go on nice holidays as a couple, etc. Whose position would you rather be in?
You've already said that as a family you still do lots of things together, family gatherings, Christmas, Easter, birthdays, etc. So, I think you're going to have to accept the fact that your tradition of UK road trips with the GCs, as enjoyable as they were at the time, have now come to an end, as the families/children obviously want something different. Maybe, at some point, you will be invited to go with them all again, but perhaps they simply want to get out of the habit of everyone always having to go away together.
As an aside, I remember when our kids were growing up, we were caught up in the situation of having 'traditions' with our in-laws (both sets), and, frankly, after a few years, particularly as the kids were getting older, we really felt we wanted to get out of the habit and do our own things, to please ourselves. Remembering that, now we're GPs ourselves, with a very close relationship with our ACs & GCs, I've made a concerted effort not to encroach on their lives, and not to expect too much from them, as I know young families like to do their own things, and to make traditions of their own.
I hope you can overcome your upset/anger, and see it as a new phase in family life now the children are all getting older.
Book yourself a nice holiday with your DH, and enjoy the freedom of not having to consider what others might want to do.