I was thinking BNP and Communists to be honest, that direction really.
Well the Communist
had seats in 1950, although lost both in the General Election that year. If they focused on holding their existing seats rather than extending their outreach (they stood five times as many candidates than in 1945) they might have been able to hold west fife, but it would have been close at best. The beginning of the Cold War really crippled the party.
In terms of racists, the NF was doing well in the seventies for a brief time. Had they prevented entryism by the Greater Britain Movement (out and out Nazis) they would have been more preferable to the electorate, especially if they had a popular figure behind them.
Although I can't see either ever taking power, or even coming close to the level of the Liberal Democrats barring some national economic disaster. The best is successful entryism into the Conservatives or Labour, such as the League of Empire Loyalists or Militant Tendency respectively.