Ideologically the UK Unionist Party is an integrationist party which, unlike most Northern Irish unionist parties, believes that Northern Ireland should be governed from London with no regional home rule government and parliament. The UKUP is outspoken in its opposition to the Republic of Ireland possessing any participative role in the governance of Northern Ireland. It is also highly critical of the British Labour government of Tony Blair for allowing Sinn Féin to participate in Northern Irish government prior to IRA fully disarming. The party also opposed the re-organising of policing in Northern Ireland, which saw the controversial Royal Ulster Constabulary (RUC) evolving into the Police Service of Northern Ireland (PSNI).
McCartney, along two other members won seats on the 1996 Forum. The UKUP (as well as the Democratic Unionist Party (DUP) refused to accept Senator Mitchell as chairman of the multi-party talks and tried to obstruct him in the undertaking of his tasks. In July 1996 the UKUP withdrew from the multi-party talks in protest at the way in which the stand off at the Orange Order parade at Drumcree was handled. When Sinn Féin entered the talks in September, 1997 the UKUP and the DUP left them in protest.
At the 1997 General Election McCartney was re-elected as an MP. The party opposed the April 1998 Belfast Agreement and campaigned against the establishment of a Northern Ireland Assembly, in which they were unsuccessful. It did contest the election for the assembly however, in which the party managed to win five seats.
In December, 1998 the party split over the issue of Sinn Féin taking up its seats in the power sharing executive without prior Provisional IRA decommissioning of weapons. McCartney censured his four Assembly colleagues in their absence over this split and in January, 1999 they all left the UKUP to form the Northern Ireland Unionist Party leaving McCartney as the sole UKUP representative in the assembly.
At the 2001 General Election McCartney lost his seat in the House of Commons to the Ulster Unionist Party. In the Northern Ireland Assembly Election, 2003 he was only narrowly re-elected to the Assembly. The party, like most other small parties, suffered a substantial decline in the election, holding only one seat. The rival Northern Ireland Unionist Party created by breakaway MLAs, lost all of its seats.