"It won't stop me going back in the water and it shouldn't stop anyone," he said, "I was just in the wrong place at the wrong time."
Mr Thomson was surfing off the coast of Bantham in South Devon when the 3ft-long shark - thought to be a smooth hound shark - grabbed his leg. Mr Thomson, a teacher, fought back valiantly. Deciding he didn't need a bigger surfboard, he bashed the shark on the head and it swam off.
Mr Thomson said: "I turned round and saw this little shark was on my thigh and wriggling its head side to side.
"I hit it on the head and it swam off. My hand was cut to pieces."
Photographs showed a clearly bloodied thumb and grazes on his knuckles.
He said that had it not been for his thick winter wetsuit, his injuries could have been far worse.
As it was Mr Thomson also suffered a "sizeable bruise about three inches across".
Experts said they believed it was the first time a surfer had been attacked by a shark in UK waters.
Mr Thomson told the BBC that when he arrived home, his wife had initially disbelieved him.
"I went home and told my wife I was late because I had been bitten by a shark," he recalled. "She said 'I've heard that one before', but it was true."+
Mr Thomson said pupils at Kingsbridge Community College, where he teaches, had bought him shark ties and have dubbed him Sharkbait and Nemo.
"I have never caught any fish while fishing but the biggest one I've ever caught attached itself to my leg," he said.
Marc Dando, a wildlife publisher and illustrator, said he thought the shark was probably a smooth hound shark.
The smooth hound, found in the eastern Atlantic Ocean from the British Isles to South Africa and in the Mediterranean, can grow up to 6.5ft but typically come in at no bigger than 5ft.
Mr Dando said: "It [the attack] would be a shock because all sharks have powerful jaws.
"All sharks can be very territorial. It was probably just telling the person to go away and struck out."
Ali Hood, director of conservation at the Plymouth-based Shark Trust said the small shark "would likely have been disorientated" by the "turbid, dynamic water" of the river mouth.
"British waters are home to a wide diversity of sharks with a number of coastal species such as smooth hound, tope and cat sharks often reported by beach goers and water users," she said.
She said she had not heard of a shark of any sort biting a surfer in British waters.