This is a very difficult question, English itself isn't a "single language" in a sense and was instead throughout history has gone through various stages.

Firstly there was Old-English also known as Anglo-Saxon. Which is again, divided into even more stages, some of which we know more about than others. Such as Prehistoric English, which only exists so far in epigraphic evidence^[1]. Historians are conflicted on where this strain of English came from, but the main two are that it came from either Frisia in the Netherlands, or from Scandinavia. Neither of these two theories however can be proved without further archaeological excavations.

This then heads into Early Old English, of which we have evidence from Anglo-Saxon written sources, such as the Anglo-Saxon Chronicles ^[2] and Beowulf ^[3]. According to the Chronicles, the Britons, requested the aid of Rome against the Picts in 449 but were rejected by them and instead turned to the Angles. Who in turn came to the Britons aid in return for land in the South of England. The Angles at the time were led by Hengest and Horsa, so these two are the "instigators" of bringing Old English to Britannia, but it could also be attributed to Wurtgern, the king of the Britons who had invited them to Britannia. In terms of Old English becoming an official language, following the original gifting of land many settlers from Mainland Germania headed to Britannia and set up seven kingdoms (The Heptarchy.) They originally spoke Prehistoric English, but this eventually evolved into Old English this spread throughout what would later be known as the Kingdom of England planting it firmly as the official language, or lingua franca of the area

However as this is not what we know as "English" today, the next step was after 1066, following the Norman invasion Middle English was formed. While English had been evolving earlier due to the Danelaw in northern England it wasn't until the Norman conquest that it really supplanted itself from Old English, as the Norman kings replaced the top part of both religious and political positions with Old French speakers. This led to "Anglo-Norman", which was used primarily by the upper classes and clergy becoming the literary norm, meaning this is what most evidence at the time is written in. While this didn't change the language immediately and most would have continued speaking their dialect of Old English, it did have an affect over many years. But not to the extent of becoming "English" and was mostly more French as many higher ups spoke Norman with the average person speaking a warped version of Old English. This can be seen in the Ormulum ^[4].

This then later evolved into Early Modern English around the 15th or 16th century, which was following large urbanisation of the populace (mainly moving to London) led to a more recognisable version of English as well as the printing press leading to more literacy meaning a more singular language opposed to the dialectical old and middle version of English. Examples of this would include William Shakespeare's work.

Finally what we could call "Modern English" in Britain, would be seen after the invention of the Dictionary, firstly in 1604^[5], but mainly in 1755 following the printing of Samuel Johnson's Dictionary of the English Language. Finally formalising the language and making what we know as "English".

So therefore, this question isn't a simple one, as many different people could be seen as bringing English to Britannia, and formalising it.

^[1] Prehistoric English Runes ^[2] [Anglo-Saxon Chronicles] (http://www.britannia.com/history/docs/asintro2.html) ^[3] [Beowulf] (http://www.bl.uk/collection-items/beowulf#) ^[4] Ormulum ^[5] Table Alphibeticall

A good book to read on this would be The Story of English by David Crystal.

For more information head over to (r/Linguistics) or (r/asklinguistics)