UKIP appeared to be a UK political party, fighting UK and EU elections, but it was really a single-issue pressure group within the Conservative Party, which was tainted by nutcases and racists. It became even more troubled by nutcasery and racism once the referendum was won, because their single issue seemed won but actually wasn't.
The Brexit Party is a political party specific to the EU election, standing on the single issue of seeking Brexit. It will probably then morph into single-issue pressure group within the Conservative Party, tainted by nutcases and racists.
Simples.
I was never willing to vote for UKIP in UK or EU elections, as they were too crazy and vile, but I am prepared to treat the EU elections as a second referendum, and just vote 'Brexit'.