Denis is the LGBTIQA+ Greens' Treasurer. This is his third year on the committee and second in the role of Treasurer. In addition to the responsibilities of Treasurer, he runs the liberation group's website and contributes to its social media presence. He is also the Treasurer of South East Essex Green Party and was Acting Chairperson of the party for several months in 2017.
Denis has campaigned on environmental issues since 2001, most notably in a successful campaign against the widening of Priory Crescent in Southend-on-Sea and an unsuccessful one against the expansion of Southend Airport.