Ten largest UK publishers

The ten largest book publishers in the UK (ranked by sales value in 2007) with some of their principal imprints. Nielsen BookScan: [1] [2]
From Wikipedia

1. Hachette Livre (UK) £299.8m (16.6%)
Headline; Hodder & Stoughton, Sceptre; Little, Brown, Abacus, Sphere,
Piatkus, Orbit, Virago; Orion, Weidenfeld & Nicolson, Gollancz, Phoenix, Everyman; John Murray; Octopus, Cassell, Hamlyn, Mitchell Beazley, Philips; Orion Children's Books, Hodder Children’s Books, Orchard Books, Franklin Watts, Wayland, Hodder Education, Chambers Harrap
2. Random House (UK) £263.4m (14.6%)
Century, Hutchinson, William Heinemann, Arrow; Chatto & Windus,
Jonathan Cape, Harvill Press, Secker & Warburg, Vintage, Pimlico, Bodley Head; Transworld, Doubleday, Bantam Press, Black Swan, Bantam, Corgi; Ebury Press, BBC Books; Virgin Books, Black Lace, Nexus, Cheek; Andersen Press
3. Penguin Books £177.3m (9.8%)
Penguin, Hamish Hamilton, Allen Lane, Michael Joseph, Viking, Rough
Guides, Dorling Kindersley, Puffin, Ladybird, Warne
4. HarperCollins £142.7m (7.9%)
HarperCollins, 4th Estate, Avon, Voyager, Collins
5. Bloomsbury £74.7m (4.2%)
Bloomsbury, A&C Black
6. Pan Macmillan £61.4m (3.4%)
Pan Books, Picador, Macmillan New Writing, Macmillan, Boxtree, Sidgwick
and Jackson, Tor (UK), Kingfisher
7. Oxford University Press £33.1m (1.8%)
8. Pearson Education £32.3m (1.8%)
9. Simon & Schuster (UK) £33.1m (1.5%)
10. Egmont £24.9m (1.4%)

A number of distinctive smaller independents - led by Faber & Faber [3, 4] - have formed an alliance to share promotion and administration:
Faber Independent Alliance £39.1m (2.2%)
Faber & Faber, Atlantic Books, Canongate, Granta Books, Icon Books,
Portobello Books, Profile Books (including Serpent's Tail), Quercus Publishing, Short Books

References
1. Alison Flood, Liz Bury, Joel Rickett and Philip Stone, Hachette steals the show, The Bookseller, 24 January 2008
2. Edward Russell-Walling, The Sunny Side of the High Street, Publishers Weekly, 24 March 2008
3. Edward Russell-Walling, Where 'Every Book Counts', Publisher's Weekly, 17 March 2008
4. Independent Alliance, Faber & Faber