LONDON—Morrisons has agreed to a takeover led by SoftBank owned Fortress Investment Group that values Britain’s fourth-largest supermarket chain at £6.3 billion ($8.7 billion) and tops a rival offer from a U.S. private equity firm. The offer from Fortress, along with Canada Pension Plan Investment Board and Koch Real Estate Investments, exceeds a £5.52 billion unsolicited proposal from Clayton, Dubilier & Rice (CD&R), which Morrisons rejected on June 19. However, it was less than the £6.5 billion asked for by top 10 Morrisons investor JO Hambro last week. Shareholders will get to vote on the Fortress offer, which gives the supermarket chain an enterprise value of £9.5 billion once its net debt of £3.2 billion is taken into account. Under British takeover rules CD&R has until July 17 to come back with a firm offer. CD&R declined to comment. Analysts have also speculated that other private equity groups and Amazon, …