https://ico.org.uk/media/action-weve-taken/decision-notices/2017/2013939/fs50610253.pdf
“83. The term “vexatious” is not defined in the FOIA. The Upper Tribunal (UT) considered the issue of vexatious requests in the Information Commissioner v Devon CC & Dransfield (UKUT 440 (AAC), 28 January 2013).1 The UT commented that “vexatious” could be defined as the “manifestly unjustified, inappropriate or improper use of a formal procedure”. This definition clearly establishes that the concepts of proportionality and justification are relevant to any consideration of whether a request is vexatious.”
