Original sentence: The new UK Independence Party (UKIP) was a major advocate of a further referendum on continued membership of what had now become the European Union, and the party’s growing popularity in the early 2010s - culminating in their being the most successful in the 2014 European Parliament election in the United Kingdom
Edited sentence: The new UK Independence Party (UKIP) was a major advocate of a new referendum on continued membership of what had now become the European Union, and the party’s growing popularity in the early 2010s - culminating in their victory in the 2014 United Kingdom European Parliament elections