An extract from The Dominance and Monopolies Review - 7th edition
Market definition and market power
The assessment of market definition and market power in the United Kingdom is consistent with EU law. The UK competition authorities and courts are required to interpret the provisions of the Act consistently with EU competition law wherever possible, and to have regard to relevant decisions and statements of the European Commission.
One difference between EU and UK law is that under the Chapter 2 prohibition there is no need to show a cross-border effect, and no minimum market size threshold: a 'dominant position' refers to a dominant position in the United Kingdom or any part of the United Kingdom. This means that dominant positions can be found even for small suppliers in small markets.