Oh dear Raggykins you don't know much about "live firings" of missiles do you? Suggest that you look at the performance specs for the Trident II and then you may (although I rather strongly doubt it) realise what acceptable test parameters might be and how slight the margins might be for aborting the test flight of a missile.

But if you really want to frighten yourself have a little trip exploring how long it has been since the Soviet/Russian Navy has successfully carried out the launching of a ballistic missile from one of their submarines (Having said that they did recently successfully test one of their new land based ballistic missiles, but that is a much simpler process).