Secret Adventures: Kayaking the River Thames in London

In the middle of a London heatwave (it's 32C, honestly), I mop my brow, slip-slap-slop and head down to the shingle-and-sand beach below the Trafalgar Tavern, one of the maritime-flavoured watering holes in Greenwich, a quaint riverside district and UNESCO World Heritage site soaked with nautical history. As crowds drift to the nearby Cutty Sark – a 19th century sailing clipper-cum-visitor attraction that ferried tea from China and wool from Australia – I'm gearing-up for my own (slightly) epic voyage: an 18-kilometre kayaking trip through the heart of London.

The tour is with Secret Adventures, a company whose outdoorsy activities showcase the British capital in a new, invigorating light.

Our 10-strong group, led by guides Harry and Chris, paddle out into the River Thames in a fleet of brightly-coloured kayaks, one of which I share with Kyra, a young London woman who, being much lighter than me, is seated in the front; I'm at the rear, steering, using foot pedals to control the kayak's rudder. Though you must be reasonably fit to make this journey – it's about three hours' kayaking in total and the muscles in your back, arms, shoulders and thighs certainly get a decent work-out – it's actually not too taxing. Tours only run when the tidal flow is in a favourable direction and the forces of nature tend to buffer you along this snaking waterway, along a similar bending route that the ancient Romans took when they sailed upriver and founded Londinium in AD43.

While it's easy to daydream as you kayak along, you must stay alert as things occasionally get choppy, especially when other vessels pass: barges carrying cargo and rubbish, Thames Clippers waterbuses and high-speed RIB boats that tear along, James Bond-style. There are also moored boats and glossy yachts to steer round and the arches of almost 20 bridges to dip under, not least Tower Bridge, the first and most bewitching of all we see today.

Floating past this and other famous sights – the Tower of London, Shakespeare's Globe, St Paul's Cathedral, the Shard and London Eye – your first instinct is to grab your camera or smartphone. But we've been urged not to carry them in case they fall into the toffee-hued river (you can safely store them, along with refreshments and water, in your kayak's small, easily-reachable cabin). In any case, our guides, armed with waterproof cameras, take plenty of pictures of us, which they'll email later, free of charge.

Being cameraless is genuinely liberating and means you can really absorb the magic of London – and its historic and ever-evolving cityscape – without worrying about securing that perfect image for your social media network. When we're not posing for photos, or receiving technical advice on our kayaking skills, we listen as Harry and Chris reveal intriguing nuggets about London and "hidden" waterfront spots we'd otherwise have breezed past without a second glance.

For instance, near the old royal dockyards of Deptford, not far from Greenwich, Harry had pointed out a small, tree-fringed Elizabethan property where it's said the playwright, Christopher Marlowe, was murdered in 1593. Rumour has it he was a double agent for both British and Dutch states – and he may have also penned some of William Shakespeare's plays. On the other (north) side of the river, Chris alerted us to The Grapes, a secluded Limehouse pub that the great thespian, Sir Ian McKellen, co-owns and lives beside. He is currently playing King Lear in London's West End.

A little further on, outside The Prospect of Whitby pub, there's a hangman's noose – placed in homage to a hard-line 17th century judge who would sentence river criminals to death. Later, after passing the majestic neo-Gothic sprawl of the Houses of Parliament, and the site of the former Millbank Prison, Harry shows us a set of river-bank steps that prisoners took to board convict ships bound for Australia.