Adds offer details in paragraph 2, context in paragraph 3

July 2 (Reuters) - Britain's Spectris SXS.L has accepted an improved takeover offer from U.S. private equity firm KKR KKR.N, valuing the scientific instruments maker at about 4.7 billion pounds ($6.46 billion), including debt, the companies said on Wednesday.

The offer values Spectris at 40 pounds per share, including dividends, and is a 6.3% premium to Advent's 37.63 pounds per share offer, marking Britain’s largest takeover deal so far this year.

Britain's cheap valuations have lured overseas buyers, adding to the challenges facing London's stock market, which is already under pressure from venue shifts and weak IPO activity.

Spectris said its board has decided to withdraw its recommendation of Advent's 4.4-billion-pound offer in favour of the higher bid from KKR, which had made two prior takeover proposals for the London-listed firm.

($1 = 0.7281 pounds)

(Reporting by Raechel Thankam Job in Bengaluru; Editing by Eileen Soreng and Sherry Jacob-Phillips)

((Yamini.Kalia@thomsonreuters.com;))