Roy was deputy head of the Home Office’s Research and Statistics Department until 1993, since when he has focused on international prisons issues. He undertook a study of the prison systems in central and eastern Europe for HEUNI (the United Nations-affiliated European criminal justice institute), published in 1996. After a consultancy with the United Nations he jointly conducted assessments of the two prison systems in Bosnia and Herzegovina for the Council of Europe, published in 1998 (Walmsley & Kriznik) and 1999 (Walmsley & Nestorovic). In 1998 he produced the first edition of the World Prison Population List (published by the Home Office in 1999).
In 2000 Roy became an Associate of the International Centre for Prison Studies (ICPS) for whom he introduced the World Prison Brief - the online database of information on prison systems and prison population levels round the world. He conducted a second study of the prison systems in central and eastern Europe for HEUNI (published 2003) and jointly conducted an assessment of the prison system in the former Yugoslav Republic of Macedonia for the Council of Europe, published 2006 (Walmsley & Kriznik).
Roy continues to direct the World Prison Brief, now hosted by ICPR, and to produce the World Prison Population List (12th edition published by ICPR in November 2018). He also produces the World Female Imprisonment List (4th edition 2017) and the World Pre-trial/Remand Imprisonment List (3rd edition 2017). He co-authored ‘Imprisonment Worldwide: The Current Situation and an Alternative Future’ (Policy Press, 2016, with Andrew Coyle, Helen Fair and Jessica Jacobson).