As a legitimate democratic party has the B.N.P, not the right to promote it's policies without violent protest?

Surely the place to express ones opposition to the B.N.Ps extreme right wing racist policies is at the ballot box and not in the streets. Northern Ireland is a prime example of how a society can be destroyed by the denial of political expression through violent suppression. Violent actions always creates even greater violent reactions and so the violence escalates into near civil war. Has the criticism of the B.N.P, by the other three major parties given a certain credibility, if not justification for the violent attacks on a party which is, after all, participating in the democratic system of politics?