The Way Ahead, without a doubt. Allegedly was shown as a training film at Sandhurst for many years afterwards.
Another low-budget one which I managed to get hold of on DVD recently, is Overlord (1975). Was produced & technical advice provided by the IWM, so you know the accuracy is good for that timeframe. Features a very young Nicholas Ball (Hazell) in his first film role. Check it out on Amazon.