Probably the best known was when the Thomson Travel Group acquired the Horizon Travel Group in 1988/89. At the time Horizon was the third biggest group & included Orion Airways, Horizon Holidays, HCI, OSL, Wings, Blue Sky and Horizon Travel Centres. It was the biggest merger ever in the travel industry and had to be approved by the Monopolies and Mergers Commission.
You can find plenty of info on Google but here's a couple of links to start you off:
http://www.competition-commission.org.uk/rep_pub/reports/198 9/234thomson.htm
http://www.holidaytruths.co.uk/viewtopic.php?p=889