I don't think it was 'mostly' about immigration, but there were other reasons as well. Of course I don't deny that immigration was a reason for many people to vote leave, but I don't think it is as cut and dry as you suggest.
For example this poll from the referendum shows that the no. 1 reason people voted to leave was 'decisions about the UK should be taken in the UK'
For example this poll from the referendum shows that the no. 1 reason people voted to leave was 'decisions about the UK should be taken in the UK'
http://lordashcroftpolls.com/2016/06/how-the-united-kingdom-...
Edit: typo