After moving to the United Kingdom, he continued with his passion for Scrabble and took part in local Scrabble competitions. He also excelled in his education including the result of 5 As in his A-levels while studying at Steyning Grammar School.[2] He began his medical studies at the University of Cambridge and later transferred to the University of Oxford, where he completed his clinical studies. He chose to work as a locum consultant pediatrician for a period of two years at Worthing Hospital. He then started working at Swindon Hospital.[2]
Harshan regularly participated in various international Scrabble competitions representing the United Kingdom including the World Scrabble Championship in 2001, 2005, 2007 and 2009. He reached a career high after emerging victorious as the National British Scrabble Champion at the UK National Scrabble Championship in 2003.[2] He registered the highest ever scoring word in Scrabble with the word "Kreuzers", scoring 329 points. He emerged as runner-up to Australia's David Eldar in the best-of-five finals during the World Scrabble Championship 2017 and Lamabadusuriya went into the final ranked number 16 in the world.[3][4] During the 2017 Scrabble World Championship, he maintained an almost perfect run throughout the tournament. This included a victory over 1993 Scrabble World Champion Mark Nyman in the quarterfinals, only lose to Eldar in the final 3-0.[5] He collected £3000 as a cash reward for his second-place finish in the competition. He competed in the World Scrabble Championship 2019 where he qualified for the knockout round in the quarterfinal stage, but he was eliminated with a 2-1 defeat to USA's Peter Armstrong.[6]
Harshan competed in the 2023 edition of the World Scrabble Championship where he achieved second-place in the tournament after emerging as runner-up to David Eldar.[7] In January 2025, he won the UK Open Scrabble Championships, receiving a cash prize of £500 prize after winning 21 of his 27 games throughout the competition.[2][8] He described that the game-defining word "backstory", which he had played during a critical moment of the final, as a eureka moment.[9]