I understand your qualms (my own parents have the same ones, and in fact the memory of the SDP split was one of the things which made me put off joining for a few years). However, it wasn't the SDP split that helped Thatcher to get in - they didn't form until 1981.
In this election it's definitely *NOT* going to be a Tory landslide - the very best they can hope for is to form a minority government. Have a look at http://ukelectiontrend.blogspot.com/ for example - it's just impossible for *any* party to get more than the tiniest majority this time round, and a Labour/Lib Dem coalition looks the most likely result.
Remember as well that the Lib Dems are taking votes not just from Labour but from the Tories - the Conservative share in the polls has *plummeted* since the debates, since people who were thinking of voting Tory. In your own seat of Holborn And St Pancras, for example, the anti-government vote will likely go to the Lib Dem candidate, who has a small but real chance of beating Dobson, so those who were thinking of voting Tory will almost certainly now vote for Jo Shaw.
(And as someone who unfortunately has Griffin as an MEP, I agree it would be terrible for him to get in in Barking. In the case of the BNP the important thing is just to get people out and voting at all - I've taken part in several Hope Not Hate events to try to get out the anti-BNP vote, and there I don't care who they're supporting so long as it's not the BNP. )
None of this is to try to change your mind - you should vote for whoever you want to win in your seat - just to say that you shouldn't worry unnecessarily. This isn't 1983, and the very real concerns that existed then don't exist now...