25% market share isn't 'the definition' of a monopoly in the UK, as far as I'm aware (IANAL or in any way an expert in this).
It's one of the necessary (but not sufficient) conditions for the OFT to refer a merger to the Competition Commission for investigation under the Enterprise Act[1], if the result would have greater than 25% market share. The other main conditions being that it believes the result would be "a lessening of competition", that it's an important market, etc. That doesn't suddenly make every company with >25% market share a monopoly.
The nearest I can find for the definition of a monopoly, for the purposes of investigating a company on its own (rather than a merger) is "a dominant position in a market"[2], the wording imported from Article 102 TFEU[3].
It's one of the necessary (but not sufficient) conditions for the OFT to refer a merger to the Competition Commission for investigation under the Enterprise Act[1], if the result would have greater than 25% market share. The other main conditions being that it believes the result would be "a lessening of competition", that it's an important market, etc. That doesn't suddenly make every company with >25% market share a monopoly.
The nearest I can find for the definition of a monopoly, for the purposes of investigating a company on its own (rather than a merger) is "a dominant position in a market"[2], the wording imported from Article 102 TFEU[3].
[1] http://www.legislation.gov.uk/ukpga/2002/40
[2] http://www.legislation.gov.uk/ukpga/1998/41
[3] http://en.wikipedia.org/wiki/Article_102_of_the_Treaty_on_th...