In a development that seems to suggest that trying to persuade large companies to throw out their PABXs and switch to Centrex service from a public exchange at this late stage may be rather like asking them to throw out their mainframes and switch to a bureau, British Telecom has postponed indefinitely launch of its Centrex service. The company has bought an AT&T System 85 exchange adapted for Centrex from the AT&T-Philips Telecommunications BV venture, and had planned to launch its service next month. Mercury Communications launched a Centrex service in May, but is not so far thought to have picked up many customers for it.