During the late 1990s Balfour-Oatts began to deal in Modern British Art, which was exhibited alongside German Expressionist paintings and woodcuts, placing works in public collections such as the Moritzburg Museum, Halle, Germany and the National Portrait Gallery, London, amongst others. ARCHEUS moved to 3 Albemarle Street, London (2000–2008) and embarked upon a programme of Modern British and International contemporary art, curated by Balfour-Oatts. In 2005, Balfour-Oatts curated a comprehensive exhibition of William Scott's graphic work and published "William Scott: A Survey of His Original Prints" , which has become the standard reference work and de facto catalogue raisonné of William Scott's printed works. Other notable exhibitions included the first commercial one-man show of Dan Flavin's work to have been held in London, "Nothing As Full As The Air", and "The Unseen Hand: Minimalism and Anonymity", featuring rare works by Donald Judd, Carl Andre, Robert Ryman and Agnes Martin in 2005 and 2006. In 2007, Balfour-Oatts became well known for his part in events that would lead to the reclassification of Andy Warhol's famous 1968 Brillo Boxes by the Andy Warhol Art Authentication Board. Four years earlier, Balfour-Oatts had visited the elderly curator and former director of the Pompidou Centre, Pontus Hultén, at his home in France. In what was seen as a feat of art-dealing detective work  Balfour-Oatts negotiated the purchase of the last 22 Brillo Boxes remaining in Hultén's collection. These boxes had, according to Hultén, been exhibited in his celebrated 1968 Warhol exhibition held at Stockholm's Moderna Museet, of which institution he had also been director. Balfour-Oatts consulted the Andy Warhol Art Authentication Board prior to completing the purchase, and duly received 22 certificates of authenticity, confirming they were indeed original works by Warhol. 12 of the boxes were sold to the American collector Don Fisher, founder of The Gap, and later presented to San Francisco Museum of Modern Art. The remaining 10 works were sold by Christie's to Warhol's former dealer Anthony d'Offay, who presented the works to the Scottish National Gallery of Modern Art. After Hultén's death, it became apparent that most of the Brillo Boxes may not have been manufactured in 1968, and had possibly been made later, under Hultén's direction, with, Hultén claimed, Warhol's permission.