The Tories would sooner ally with the Greens than the BNP. They seriously do not need the kind of PR that party would bring.
And anyway the BNP are has-beens anyway. So 2009.
As for personhood, UKIP don't seem to be making a big deal about it. Perhaps if the Tories have to rely on DUP for supply and confidence, or if Nadine Dorries inexplicably becomes a kingmaker.
Also, I'm pretty sure that a minority government is much more likely than a full-blown coalition. I can imagine a gaggle of Kippers giving supply; but a full-blown UKIP-Tory coaliion would be poisonous to both parties.
It of course depends on whether tories need the far right. Some polls have the far right doing well enough to force a progressive minority government if the right doesn't unite.
I wonder if a grand coalition might end up as a possibility, especially if both UKIP and especially SNP do well enough to muddle possible majorities for either side but still aren't seen as trustworthy coalition partners?