Ultimately the truth is that there are no concrete plans to create a Single European Army. It is the personal aspiration of Jean-Claude Juncker, the current president of the European Commission, and some other European politicians — but currently appears to be no more than that. 

What is also important to note is that this is not something which could be implemented against the will of the British people; as the European Act 2011 http://www.legislation.gov.uk/ukpga/2011/12/section/6 requires that the British Government hold a referendum in the event this proposal is tabled. Not only this, but several other EU members have received the idea less than enthusiastically, such as the Czech Republic, Romania, Slovakia and Poland — as can be seen in this article http://www.euractiv.com/section/global-europe/news/uk-central-europe-frown-at-juncker-s-european-army/ from March 2015.

Through Nato and inter-governmental treaties the UK already partakes in many cooperative military exercises with other EU countries, and indeed the UK and many other EU countries (plus Canada and the US) are bound to act as one in the event of an attack on one of the Nato countries.

We can easily conclude that the chances of the UK being dragged into a single European Union Army against its will are zero.