Billings­gate fish mar­ket is London’s old­est whole­sale mar­ket. It offi­cially opens at four o’clock in the morn­ing when a bell sig­nals the start of trade and it closes at half past nine. But the day’s work starts well before trade begins and con­tin­ues through the morn­ing on and off the mar­ket floor. The archi­tec­ture of Billingsgate…