* Before English was called "English": English evolved from Old English, spoken in England from roughly 450 to 1150 AD. Before that, it was called "Anglo-Saxon" and was heavily influenced by Old Norse, Latin, and other Germanic languages.
* Before English was a language: The languages spoken in Britain before the arrival of Germanic tribes (Anglo-Saxons) were Celtic languages, such as Brythonic (which evolved into Welsh, Cornish, and Breton) and Gaulish.
* The very first language: It's impossible to know for sure what the very first language was. Language is a constantly evolving phenomenon, and there are no written records from the earliest periods of human history.
So, the answer depends on what you're asking. If you're looking for the language that directly preceded English, it was Old English (or Anglo-Saxon). If you're looking for the language spoken in Britain before the Anglo-Saxons arrived, it was Celtic languages. If you're looking for the very first language, that remains a mystery.