LONDON, Sept 12 - Britain said on Tuesday the next Brexit talks had been postponed until Sept. 25 to give "negotiators the flexibility to make progress in the September round."Earlier, Brussels diplomats told Reuters a new round of talks between Britain and the European Union had been delayed until the end of the month to let Prime Minister Theresa May make a key speech on Sept. 21."The UK and the European Commission have today jointly agreed to start the fourth round of negotiations on September 25," a government spokesman said in a statement."Both sides settled on the date after discussions between senior officials in recognition that more time for consultation would give negotiators the flexibility to make progress in the September round."