LONDON (Reuters) - Global consulting firm McKinsey & Co signed a long-term lease for a new office in London, signaling its commitment to Britain at a time when the country's coming withdrawal from the EU means some groups are relocating staff away from the UK.

McKinsey announced a deal on Wednesday to take 100,000 sq ft (9,300 sq meters), plus an option to take up to 130,000 sq ft, in the Post Building in London's West End from 2018.

The firm will be moving from its Jermyn Street offices in central London where it currently has 115,000 sq ft of space, a spokesman said.

"It represents a renewed commitment to London and the UK," McKinsey's UK and Ireland managing partner Vivian Hunt said in a statement. "The Post Building gives us the flexible and modern office space we need for our growing operations."

It did not say exactly how long the lease would last.

McKinsey's commitment to Britain comes as companies such as Google <GOOGL.O> and Facebook <FB.O> have said they plan to expand in the UK, defying predictions that international companies would shrink in the country as leaving the EU will make it a less attractive place to invest.

But some companies are turning away from Britain. HSBC <HSBA.L> said on Wednesday it would move staff generating around 20 percent of its trading revenue to Paris following Brexit.

British Prime Minister Theresa May has said she will trigger the process to leave the EU by the end of March, opening two years of negotiations.

(This version of the story was corrected to show option is for up to 130,000 sq ft, not a for a further 130,000 sq ft)

(Reporting by Sarah Young; Editing by Adrian Croft)