Obama Starts UK Visit, Security Issues Paramount

* Obama arrives in Britain early to avoid ash cloud

* Royal pomp awaits

* Libya, Arab Spring among topics for Obama, Cameron

By Steve Holland and Jeff Mason

LONDON, May 24 (Reuters) - U.S. President Barack Obama on Tuesday begins a visit to Britain where he and Prime Minister David Cameron will review NATO action to help end conflict in Libya and Western policy towards uprisings in the Arab world.

He and Cameron will also launch a joint U.S.-British group to tackle national security challenges together.

As a precautionary measure Obama flew to London on Monday night instead of Tuesday morning because of fears that a volcanic ash cloud from Icelandic could drift over Ireland and prevent his Air Force One jet from flying.

That forced him to cut short a cheerful visit to Ireland, where he sipped a pint of Guinness in the village of Moneygall to celebrate ancestral roots there and used a speech in Dublin to lift Irish spirits bruised by a severe economic downturn.

In London, Obama will experience some of the celebratory pomp put on by the British royal family while on a four-nation trip that will include a Group of Eight summit in Deauville, France, later in the week and conclude with a stop in Poland.

On Tuesday he will attend a formal arrival ceremony at Buckingham Palace laid on by Queen Elizabeth, a wreath-laying at Westminster Abbey, talks with British Prime Minister David Cameron and a state dinner hosted by the queen. Obama and his wife, Michelle, are to spend two nights at Buckingham Palace.

ASIA, MIDDLE EAST CROWD THE AGENDA

His speech at Westminster Hall on Wednesday may give him an opportunity to outline how Washington now views Europe in a crowded diplomatic agenda dominated by challenges from Asia to the Middle East.

Obama and Cameron are to announce the formation of a U.S.-British national security council to work together on international challenges and share intelligence, an Obama administration official said.

It was not developed in response to any one issue, but will help enable "a more guided, coordinated approach to analyze the 'over the horizon' challenges we may face in the future."

Obama and Cameron, who are to hold a joint news conference on Wednesday, reaffirmed their countries' close ties in a joint article that was appearing in The Times of London.

"It is a perfect alignment of what we both need and what we both believe. And the reason it remains strong is because it delivers time and again. Ours is not just a special relationship, it is an essential relationship - for us and for the world," they wrote.

A challenge for the two leaders is Libya. Obama, Cameron and their NATO counterparts launched an air campaign in March to protect Libyan civilians from Libyan leader Muammar Gaddafi in a civil war between him and rebels who now control eastern Libya.

While many Libyan civilians, especially in the east, have been protected by NATO air strikes, Gaddafi remains in power, ignoring calls from Western leaders like Obama and Cameron on him to go to permit a democratic transition.

Obama and Cameron are also expected to review the fight against Islamist militants and relations with Pakistan after the killing of al Qaeda leader Osama bin Laden by U.S. special forces on May 2 on Pakistani soil.

Obama will be looking as well for British support for his effort to advance the movement for democratic change offered by the "Arab Spring" uprisings in the Middle East. (Additional reporting by Keith Weir; editing by Mark Heinrich)