http://www.adoption.org.uk/information/step_ad option.html
some good info here - as it points out, there are very few circumstances in which it is in the childs interests to be adopted by the step parent - despite what the parents may want, the childs interests are paramount, and the court is likely to take the view that it's in the childs interest to remain the same father.
it seems to hinge on whether the dad was married to the mum or had arental responsibility. as it als point out, if you make an application, it might have unextected side effects like the father resuming contact