BNP are a party based on racial ideology that want Britain to be a white Christian society. They are an organised political party but have struggled to really make headway because they aren't well run, fall out amongst themselves and they find it hard to attract support from the electoral mainstream because of their thug image and also because fundamentally most British people find anything with a hint of Nazism distasteful and despite the BNP trying to use imagery of Churchill and Britain in the Second World War, their ideology (white society, statist, anti-homosexual, anti-liberal) is more in common with Nazism.
EDL are just a group based around hostility to Islam mostly, they aren't an organised political party, they are just a vehicle for predominantly young males that dislike Muslims.
UKIP has never been a party based around race ideology or anything like that, the party's ideology is based around hostility to the EU, it's been knocking at the fringes for a while, more recently they have switched their main thrust to immigration because it raises their profile and allows them to target more voters than just an anti-EU campaign: for several elections before they have fought on a ticket of complaining about things like European social legislation and the Working Time Directive and lobbied to remove those rules from applying to British businesses but most of the electorate is far less interested in the Europe issue than the Westminster elite is. So the thrust of their anti-EU campaign now is focused around the point of free movement of labour in the single market, hence their argument all the Europeans can take your jobs.
This has had the advantage for UKIP of allowing them more publicity and a route in to other parts of the electorate than their previously very Tory style wealthy southerners, the downside is the immigration issue attracts nasty people like moths to a flame, hence UKIP have had a series of embarrassments from individuals getting involved wanting to stand as councillors etc that have made unpleasant racial based comments: these people have just come to UKIP as they see them as a way of getting electoral success for an anti-immigation movement when really they want to go further.
You can see actually on the small sample of TSR demographic, the two types of UKIP supporters, there are some that seem to be active posters on political issues that are more the traditional UKIP demographic (ie probably if the Conservative party was Eurosceptic, they would be in that) and then there are some of the ones that are obsessed about race based issues and make multiple threads designed to stir things up against Muslims/Jews/blacks that also pledge their support for UKIP in here. I expect their presence embarrasses the more traditional UKIP supporters as the vocality of these people is what turns mainstream people off UKIP and limits their electability.
I think the problem Farage has is a problem Labour had in the 1980s when the Labour party was seen as a vehicle for some extreme left wing Trotskyists that tried to use trade unions and the Labour party as a vehicle for their own purposes, it just put people off and the Labour party really struggled to attract moderate voters as it was associated with the loony left. When Blair came in, for all that he got criticised "the Labour party isn't the Labour party, he's a closet Tory", all of those extremists disappeared from Labour and it made the party a lot more electable. The issue Farage has now, to try and break UKIP in to the Westminster scene, he will need to start to attract other parts of the electorate that associate UKIP with racist nutters. Of course this image is pushed by parts of the media as well, but the presence of the race-obsessers in the UKIP movement makes their image toxic.