Back to about 1600, you'd be fine if you know how to read Shakespeare. Before 1600, you'd notice a rather abrupt change to Middle English (a reversal of the Great Vowel Shift), and would start to have some problems that would get worse the further back you go. Middle English's cadence is very different from that of modern English, as are the pronunciations of a lot of the words. Before around 1200, you'd find two languages in England; French spoken by the nobility, and Old English (with a few French influences) spoken by the majority of the population. Old English is very different from modern English. Here's an excerpt from Beowulf:

Weard maþelode, ðær on wicge sæt,

ombeht unforht; "Æghwæþres seal

scearp scyld-wiga gescad witan,

worda ond worca, se þe wel þenceð.^^1

(The Thorns(þ) and Eths(ð) are both pronounced as "th")

If you can translate that, you'd be good in England back to around 500 (when the majority language switches to Celtic with a bit of Latin), but if you can't (which, given your credentials, I'm guessing is the case), 1200 is about as far back as you can go.

^^1 Those were lines 286a - 289b. Seamus Heaney gives the translation as "Undaunted, sitting astride his horse, the coast-guard answered, "Anyone with gumption and a sharp mind will take the measure of two things: what's said and what's done."

Edit: added a translation of the Beowulf quote