Slidin Fast. IMO there was only a referendum so that Cameron could bolster his position in his party - and he thought a 'remain' vote was a foregone conclusion.
Since promising the referendum though, things have changed. The refugee problem turned into a crisis and some parts of the media focussed on the fact that someone migrating to continental Europe could come to Britain under the 'free movement' policy. Although Turkey isn't an EU nation, there was also a lot of attention on what might happen if Turkey joined. A common theme was about 70 million Turks could be heading for Britain - as if an entire population would want to do that.
Immigration was the main issue I'm sure, though many kept their heads down for fear of being accused of racism. Expressing concerns about immigration levels is not racism. Unfortunately, racist factions do use it as a flag of convenience.