It is still unknown which correlates of physical activity behaviour (PAB) may be effective and how they may influence PAB in UK children. The objective of the current study was to generate a conceptual analysis of the correlates of PAB in UK children (5–12 years) using the input of researchers in the field of physical activity (PA experts; PAE) and other fields (non-PA experts; non-PAE). A concept mapping approach was used to identify potential (new) correlates of PAB in children, assess their importance based on rating of potential modifiability and effect, and generate a concept map depicting the associations between them. In the first (brainstorming) stage (n = 32 experts) yielded 93 correlates, including 14 new correlates not identified in previous reviews. In the second (rating and sorting) stage (n = 26 experts), 32 correlates were rated as important and a four-cluster concept map was generated including themes related to Society/community, Home/social setting, Personal/social setting and Psychological/emotional correlates. Two additional concept maps were generated for PAE and non-PAE. From expert opinion, we identified new correlates of PAB that warrant further research and we highlight the need to consider the interaction between intrapersonal and external correlates when designing interventions to promote PA in UK children.