UKIP has a lot of supporters, and they are dispersed, but that means they already have a decent base of support in most places. Consequently it would not take too great a swing of votes from the other parties to achieve wins.
Constituencies have an inbuilt bias to favour the two big parties, but coalitions are hardly unknown; and when the public is outraged by a major act such as a disregard of the people's voice, it ought not take much for a third party to have surprise victories all over the place.
UKIP's stated policy is now fulfilled, but since it hasn't been acted upon I can't believe they think they have no further role. A referendum without the fulfilment of the public's will is a half finished task.
General elections are not only won on the economy. For sure it is a high priority and usually swings the day, but the referendum has already shown that it can take second place when something vital, such as getting one's country back again, is a possibility.
Farage is a UKIP member. He may well stand as an MP, he's only relinquished the leadership. No party is one individual alone, and whilst his voice is still there he will still have a large effect anyway.
Protest voters may tend to be a small % of constituency turnout, but nothing as important as ignoring democracy has been inflicted before.
I suspect you are wrong and misjudge the extent of the outrage when you believe disgruntled Lab or Con voters will simply abstain. Some will, but others know a vital issue when they see it.
A party taking 50% of disaffected Lab and Con voters have no need to be gaining between the two voting levels. They could start near zero. Take a simplified example.
Party C = 100 voters
Party L = 100 voters
Party U = 5 voters
50% of Party C voters vote instead for Party U, 50% of Party L voters vote instead for Party U.
Party C = 50 voters
Party L = 50 voters
Party U = 105 voters
UKIP don't stand in all constituencies presently. Do you think they'll be short of volunteers and finance if Brexit doesn't happen ?