That's a huge question but fortunately I already wrote an answer about a year ago. I reposted it here. Here's a condensed version:

First: Paleo-languages that were indigenous to the British Isles

Second: Celtic languages

Third: Latin via the Roman expansion with possibly a Romance dialect surviving until the Anglo-Saxon migration.

EDIT: To your question "Was it similar to Irish and Scottish Gaelic," the answer is Yes. Common Brittonic was the ancestral Celtic language of England, which later grew into Welsh in Wales and Cumbric in Cumbria (extinct) and Cornish in Cornwall (technically extinct). Common Brittonic and the Goidelic languages (Irish and Scots Gaelic, for example) are distantly related as Insular Celtic tongues, a branch of the Celtic language family.

hi! you can find some more info about pre-Anglo-Saxon languages in England ...

in the FAQ* Why doesn't England speak a Latin-based language like other former European Roman provinces

... and a few other posts

Who inhabited the British Isles before the Roman invasion?

Who lived in Britain before the Celts?

What language would have been spoken during the building of Stonehenge?

The pre-Roman/pre-Anglo-Saxon languages of England were more akin to Welsh than Gaelic. One of the ways that they are differentiated is that Welsh is classified as a p-Celtic language and Gaelic as a c-Celtic language ... this from a shift from a mutual ancestral language so that in Scots Gaelic, the word for 'head' is "ceann" and in Welsh, it is "pen." Today, Welsh and Gaelic are not mutually intelligible.