(Reuters) - London's main index eked out modest gains on Wednesday, hours before the sealing of an initial U.S.-China trade deal, though sentiment was tempered as Washington said tariffs on Chinese goods would not be rolled back immediately.The FTSE 100 edged 0.1% higher, supported by housebuilders, after Persimmon forecast annual profit in line with market estimates and said it was in a strong position ahead of the Spring season.Its shares added 2%, while those of peers Barratt and Taylor..