British PM May to set out vision of Brexit "success" as some investors fret

* PM May to speak on Brexit

* Nissan gives Brexit ultimatum to Britain over investment

* Investors concerned Britain may end up with 'hard Brexit'

* PM May says "Brexit means Brexit"

By Guy Faulconbridge

LONDON, Sept 30 (Reuters) - British Prime Minister Theresa May will set out her vision of a successful Brexit on Sunday as investors raise growing concern that the world's fifth largest economy could face upheaval from a disorderly exit from the European Union.

The June 23 Brexit vote took many investors and chief executives by surprise, triggering the deepest political and financial turmoil in Britain since World War Two and the biggest ever one-day fall in sterling against the dollar.

May's government has tried to reassure manufacturers and banks that Britain is open for business and that it will take their views into account during the country's negotiations on new trade relations with the EU.

In a speech to open the annual conference of the ruling Conservative Party on Sunday, May will give her first big speech on Brexit since winning Britain's top job on July 13.

"The Prime Minister has been clear that we're going to get the best possible deal for Britain and for business," a spokeswoman for May said. "There are opportunities presented by Brexit, and we're going to grasp them and make a success of it."

After May's speech on "Global Britain: Making a Success of Brexit", Foreign Secretary Boris Johnson and May's Brexit minister, David Davis, will speak on the subject.

May, who quietly argued that Britain should stay in the EU during the referendum campaign, has repeatedly said since the vote that "Brexit means Brexit" but that she will not trigger the formal EU divorce process before the end of the year.

"HARD BREXIT"?

Despite warnings before the vote that Brexit would shatter economic confidence, some positive economic data and SoftBank (Swiss: SOFB.SW - news) 's $32 billion takeover of Britain's technology company ARM have stoked the perception that Britain could prosper outside the EU.

But the economic data is mixed, sterling is trading 40 U.S. cents lower than the six-year highs it reached in mid-2014 and some investors have raised concerns that their businesses could be hurt, especially by a so called "hard Brexit".

Under a "hard Brexit", Britain would abandon the EU's customs union, give up on seeking preferential access to the single market and impose controls on EU immigration to Britain.

Nissan Chief Executive Carlos Ghosn cautioned that he could scrap new investment in Britain's largest car plant unless Britain pledged to pay compensation for any tax barriers resulting from Brexit.

Jaguar Land Rover will "realign its thinking" on investment if Nissan gets a Brexit compensation deal and other automakers would need a level playing field, the head of Britain's biggest carmaker said.

"There will clearly be commentary from a variety of sources as we move forward, but we're not going to give a running commentary ourselves - nor speculate on outcomes of negotiations that haven't happened yet," May's spokeswoman said.

JPMorgan (LSE: JPIU.L - news) said in a note to clients that it would be a surprise if May did not use her appearances at the Conservative Party conference to give some guidance on her views of Brexit.

"In the eyes of many observers, the absence of detail on the approach the UK will take to Brexit is creating a vacuum which "hard Brexit" is starting to fill," JPMorgan said.

But May, who turns 60 on Saturday, has to balance an array of views on Brexit, not least from many members in her own party who are pushing for a clean break with the EU.

(Editing by Ralph Boulton)