It’s just before dawn on a damp autumnal morning, and I’m standing in what must be the coldest building in London while a man in a hat waves a live lobster in my face. If my day has started weirdly, then I guess the poor old crustacean has it worse. The creature, a handsome American of about 40, seems unimpressed by the honour of his unexpected close encounter with Barry O’Toole, inspector of fish and shellfish at Billingsgate, Britain’s biggest inland fish market.

Around Barry and the lobster, the business of the market, which moved to its present site between the A1261 and the towers of Canary Wharf in 1982 after 900 years upstream, ebbs and flows like the river it once hugged. A trolley of polystyrene boxes, silvery tails spilling from the tops, sloshes across the shiny floor behind me, narrowly missing a Chinese grandmother scrutinising the salmon heads. Men shout and wheels squeak, phones ring out as orders are placed and couriers are booked, and, at the risk of coming across a bit Emily Thornberry, I haven’t seen so many St George flags in one place since the last World Cup.

It’s a strange, crepuscular world where the working day starts at 10pm, with the first deliveries. The processors, the men whom I see deftly filleting and gutting to the strains of Radio 2 as the sun comes up over the Barclays headquarters, get in at 1am, roughly the same time as the stock from the British coast.

The merchants themselves – the men (for they are all men; I don’t see a single woman working on the market floor, though Barry assures me they do exist) who run the 130 or so businesses that rent space at Billingsgate – are in and on the phone striking deals from 2am onwards, though until the bell rings at 4am not a single fish is allowed to leave. Four hours later the place is a sea of melting ice and discarded packaging, and in the car park a large gull pecking at a salmon carcass is the only sign of life. If you’re not there by 6am, forget it.

Given this, it’s not surprising that Billingsgate is still largely a trade haunt. I pass my local fishmonger’s van on the way in. Yet it also attracts a significant number of ordinary shoppers on the hunt for specialist ingredients, or just a bit of a bargain. A couple of Japanese students in padded jackets size up the halibut with a critical eye, and I spot an African-Caribbean couple discreetly sniffing a piece of saltfish. These are people who know what they’re looking for.

But the name you hear over and over again at Billingsgate is “China”. The boys at Mick’s Eel Supply, the market’s last remaining vendor of live eels (stored in what appears to be a giant wet filing cabinet), say the Chinese are some of their best customers though they’ve yet to catch on to the delights of the jellied variety. As Barry checks a consignment of live brown crabs on their way to the airport, I am astonished to learn that they’ll be dinner in Shanghai in a little over 24 hours. The market sent 14 million tonnes of shellfish to the People’s Republic last year and its appetite for our seafood shows no signs of slowing.

Which is perhaps lucky for the merchants of Billingsgate, because, certain communities aside, we Britons today aren’t big fish fans: the average adult consumed little more than a single portion a week last year. We’re conservative, too, the same five species of cod, haddock, tuna, salmon and prawns accounting for 75 per cent of all sales. Little wonder, when most supermarkets offer little else. C J Jackson, who runs the Seafood Training School at Billingsgate, says she hasn’t seen any change in eating habits since she wrote her first fish cookbook over 20 years ago.

It may be chilly, and blokey, and annoyingly located. But I can’t help thinking that, if everyone was lucky enough to have a Billingsgate on their doorstep, things might be quite different.

Next week: Nina Caplan on drink