For some, it's visiting every football league ground, for others, tube stations. For me, by default, it's British nuclear power plants. And there's Dungeness B! The full set. I stopped to take a photograph, grateful for digital technology. I could imagine, in this day and age, that a man taking so many films of nuclear power stations into Snappy Snaps might have some explaining to do.

Through the flat, eerie landscape fringing Romney Marsh, covered in shingle, like a giant low-maintenance garden, the odd tuft of oatgrass and fan of viper's bugloss clinging on for grim death. Through the Cinque ports of New Romney, Hythe and Folkestone, and then a steep dive down into Dover, and straight up again the other side onto the clifftops. I sat on a bench at Langdon Cliffs and looked down at the ferries and catamarans waltzing around each other gracefully in Dover's port, before they passed between the harbour walls and arrowed towards the grey bluffs of Cap Gris Nez.

At South Foreland I ran out of Britain again and had to head north. Some 30 miles later, at North Foreland, I had to head west – the final turning point – along the Viking Coastal cycle trail. It skirted the base of the chalk cliffs of Minnis Bay, as if I was riding under the ramparts of some giant alabaster castle.

"Where's that?" I asked a man in Herne Bay, pointing to a town in the distance across the water.

"Southend," he replied.

"Southend," I said, in whispered awe, in much the same way I imagine that Columbus did upon sighting the New World. "I was there five months ago. Can you believe it?"

The man, without the benefit of context, seemed able to believe it quite easily.

Through lovely Faversham, and then out into the fields and orchards, the feral escapees lining the lanes groaning with fruit. I stopped to eat some succulent wild pears and then cycled on to the marshland village of Conyer, and along the levees, floating across the sunken landscape, past the ribs of eviscerated boats sticking out of the mud like dinosaur carcasses.

Gillingham, Chatham, Rochester – a sign read "London 30". At Gravesend, the Thames reappeared, not wide and majestic, as it had been the last time I'd seen it at Whitstable, before the Isles of Sheppey and Grain had obscured it, but imprisoned by concrete banks. I felt faintly claustrophobic, flushed with a sense of loss, grief even, for the absent vastness of the sea, my constant companion for almost half a year.

I passed the gigantic Bluewater shopping centre, then the Dartford Bridge and Erith. I was being sucked into London. Thamesmead, Woolwich, then there, beyond the Thames Barrier, were the towers of Canary Wharf. The Woolwich ferry shuttled back and forth across the river. I considered jumping on it and going round one more time.

Greenwich. New Cross. Red buses. Sirens. Very familiar streets now. My town. Not my town. I stopped at a red light and looked down at my bike. I thought about the places it had taken me to – across the bouncing bridges suspended in the sky, past the castles of Northumberland, to wild Cape Wrath, through the Assynt mountains, the lonely, windswept Outer Hebrides, around the majestic sweep of Morecambe Bay, the Gower peninsula, up and down the murderous hills of Devon and Cornwall, to Land's End, and, finally, Bermondsey. It seemed impossible that this piece of steel could have carried me through all that.

At Tower Bridge, a friend was waiting for me. We had a few beers at a riverside pub. He asked how the trip had been, but it already seemed like a fast-receding dream and I struggled to remember much detail.

We walked up to Blackfriars Bridge, where it had all started five months before. All I had to do now was ride across the Thames and the circle of Britain was complete. The rain had started to fall gently. I felt reluctant to cross, as if this were the best book I'd ever read and this the last precious page.

Finally, I said farewell to my friend and rode onto the bridge, feeling dazed, heading for home alongside the cycling commuters, as the rain started to fall more heavily.

• Miles this week 285. Total miles 4,625

• Mike stayed at the Zanzibar in Hastings (+44 (0)1424 460109; zanzibarhotel.co.uk).