I don't really see where there was ever a language imposed on the entire British Isles.
England’s domination over Wales, Scotland, and Ireland introduced the English language to these regions, but with the devastating consequence of the downfall of the local languages. Welsh, Scottish Gaelic, Scots, and Irish (among others) were all prohibited in education at one time or another, which possibly contributed the most to the plummeting usage of the languages. In Wales, the Welsh Not (a piece of wood with the carved letters “WN” that was hung around the children’s necks) was used in the 1800s to punish students for speaking Welsh, and beating students for using non-English languages was common throughout all of the countries. Welsh, Scots Gaelic, and Irish had inferior status to English, whereas Scots wasn’t even recognized as a separate language, and all suffered as a result. It wasn’t until the 20th century that the British government started taking steps to protect these languages, which has been met with mixed success. In all of the countries the local languages are now spoken by a minority, and are still very much secondary to English.
About other cases of modern linguistic genocide, see
http://listverse.com/2010/02/26/10-modern-cases-of-linguistic-genocide/Remuŝ
