pain > labour, pains, effort
4 Or power, be able it to remedy, 5 When the full time prefixed by destiny
prefixed > determined beforehand
6 Shall be expired of Britons' regiment.
regiment > rule, government
7 For heaven itself shall their success envy,
envy > begrudge
8 And them with plagues and murrains pestilent
murrains > pestilences
9 Consume, till all their warlike puissance be spent.