You didn't go back far enough. They were established as an anti EU party shortly after the period that the Conservatives took the UK into Europe and Labour and the unions were eurosceptic.
When Delors gave his famous Social Europe speech to the TUC in 88, the positions changed. The Tories became split between eurosceptic and pro europe, while Labour became pro EU.
UKIPs founder Alan Sked became disillusioned by the influx of conservative refugees and left the party, eventually to return in 2013 to found 'New Deal' a party set up to challenging Labour and reclaim Euroscepticism for the left wing.
Also it should be noted that Alan Sked, leftist founder of UKIP was, along with Farage, a voracious supporter of Enoch Powell. Read into that what you will.
With regard to the BNP, EDL and Britain First, absolutely. Their rank and file are hardly disillusioned Conservative voters are they. They are strongest in Labour heartland regions that have been left to rot for generations. They believe in big government and heavy restrictions on immigration. These groups are about as far away from right wing as Ryan Giggs if we actually look at what they do and say, and not what the Guardian says.
You can argue that the gates to immigration were opened by Labour, but that was during a period where Labour realised the trick to success was to follow Tory policies, but not actually be Tories. The free movement of labour is quite consistent with centre right liberal economic thought.
What is actually happening with UKIP is that they are coming full circle. They started on the left, veered right and have taken a sizeable bite out of conservative support with their EU talk. Now they have that base secured, they are going back to their roots, targeting disillusioned unionists and Labourites, talking about immigration and the NHS.
Its actually quite clever approach, but also a massive contradiction and internal conflict waiting to happen. I figure they just want some presence in the next coalition Government and will deal with issues later