Here is a clip from Wikipedia
==================================
Sovereignty

On the day of the referendum Lord Ashcroft's polling team questioned 12,369 people who had completed voting.[6] This poll produced data that showed that 'Nearly half (49%) of leave voters said the biggest single reason for wanting to leave the European Union was “the principle that decisions about the UK should be taken in the UK”.

''

Immigration[edit]

Lord Ashcroft's election day poll of 12,369 voters also discovered that 'One third (33%) [of leave voters] said the main reason was that leaving "offered the best chance for the UK to regain control over immigration and its own borders."'[8]

Immediately prior to the referendum data from Ipsos-Mori showed that immigration/migration was the most cited issue when Britons were asked 'What do you see as the most/other important issue facing Britain today?' with 48% of respondents mentioning it when surveyed.[9]

==================================
Reading this from the bottom up: Immediately before the referendum 48% mentioned immigration as a major issue and in the analysis of the votes one third mentioned it as their main reason for voting as they did.   That is a very substantial figure, even if it the case that 49% gave sovereignty as the main reason. So the initial evidence would seem to suggest that the 33% could well have been the deciding factor.

It is also the case that there are social reasons people do not always give the honest answer when surveyed, so the figures could be closer than the 33-49% would suggest. There is no need to dwell on that though, since 33% is already substantial.

No attempt to link to the page, but it appears to be from Here

If it is, it represents a very one-sided view.
Immediately before the referendum 48% mentioned immigration as a major While that looks as if it might refer to the views of the leave voters, it doesn't actually say that, and was from a poll before the vote. So will never tell us what percentage of leave/remain voters thought immigration an important point prior to the referendum.

Elsewhere in the Wikipedia article it has a heading for: Lies and misinformation which mentions only the discussion of the "side of the bus", and 'leave' dealing in "post-truth politics".
In a balanced article the heading of "Lies and misleading information" would also have covered the 'facts' put forward by Project Fear about the immediate post-referendum Armageddon which was expected if we voted to leave (but which failed to appear). No surprise this is not mentioned.
As such it seems the Wikipedia article is not a lot of use taking the discussion here any further forward.