Both countries want the UN to return senior political staff to Iraq. Britain sees UN officials helping with party meetings or caucuses to send delegates to a provisional assembly before a transfer of power to Iraqis by the end of June.
But UN officials are wary about fielding staff throughout the country and so far have advised its officials to commute to Baghdad from Cyprus or Jordan.
The UN withdrew its international staff in October after two bombing attacks on its Iraq headquarters, one of them costing the lives of 22 staff and visitors.
Since then Annan has emphasised he would not send them back unless security improves and unless the UN political role was well-defined in contributing to the country's future.
He has called a meeting for 19 January that would include the US-led occupation authority in Iraq and leaders of the US-installed Iraqi Governing Council.
On Friday, the UN ambassadors from the US and Britain, John Negroponte and Emyr Jones Parry, respectively, were due to meet Annan to discuss plans for the 19 January meeting, diplomats and UN officials said.
Although the US wants Annan to send staff back into Baghdad, it has not outlined any specific role and is reluctant to cede any power over the next six months, diplomats say.
The persuaders: John Negroponte
(L) and Britain's Emyr Jones Parry
Britain has been more outspoken in wanting the UN to play an important part in the transition, thereby legitimising the process to self-rule in the eyes of Iraq's neighbours as well as other UN countries.
It has offered to send Sir Jeremy Greenstock, its top envoy in Iraq, to the 19 January talks. But the US has not yet made clear the level of its representation, from Washington or from Baghdad. Annan, however, has said he expected the Bush administration to participate "at a senior level".
On the Iraqi side, US officials are worried about an effort by the Grand Ayat Allah Ali Sistani, a leading Shia Muslim leader, to hold elections instead of regional caucuses that would appoint a provisional government by June 30. Shias comprise up to 60% of the population.
Annan, however, has made clear that there was no time to organise elections. The issue was raised again in a 29 December confidential letter to Annan by Abd al-Aziz Hakim, a Shia, who held the rotating presidency of the Iraqi Governing Council last month.