Yes, the BNP and UKIP can elect 12 guys and watch the Labourites and Liberal Dems govern from the hard left. Good idea.
I’m sure that there are some ridings (that’s what they call districts in the UK, right?) in which the BNP or UKIP can win a plurality, and I’d probably support the candidate of one of the two parties in one of those ridings, but in most of the UK the BNP and UKIP can’t get above single-digits and it would be counterproductive to vote for them instead of for the Tory candidate. Heck, the BNP and UKIP might actually be able to form part of a governing coalition if the Conservative Party wins the most seats but is short of a majority, thereby moving the country to the right.
No, they are called constituencies in the UK. You're thinking of Canada, where election districts are indeed called ridings.