Shame about the IWM.
Often go on holiday to Dorset and always visit the Tank museum just outside of Weymouth,been about 5 times now.
Great day out and plenty to keep the kids entertained,(if they like that kind of thing) WW1 trench experience, you can climb inside some of the tanks and see what conditions were really like and if you coincide it with a day when they are doing a live demonstration around the outdoor arena even better.