。
It must be at least three years since she last came to England.
Literal
She [topic-は] this-time-before England [to-に] came [since-てから] at-least 3-years [becomes-にはなる] [should-はず-だ].
A dense temporal sentence stacking several patterns. ~てから ('since doing X') marks the starting point. すくなくとも ('at least') frames the minimum estimate. ~にはなる ('comes to be / amounts to') is a fixed pattern for rounding up time spans — '(it) becomes [time amount].' ~はずだ ('should be, must be') marks logical inference based on what the speaker knows. The compound ~にはなるはずだ together expresses 'should at least come to be X.' 英国 is the formal/literary term for the UK — built from 英 (a phonetic representation of 'En' from England) + 国 ('country'). The everyday term is イギリス, but 英国 is preferred in news, government, and formal writing.