。
She must have visited England last summer.
Literal
She [topic-は] this-time-before [genitive-の] summer England [object-を] visited [must-be-にちがいない].
~に違いない ('must be, no doubt') is a strong inferential pattern — the speaker is essentially certain based on available evidence, though not stating direct knowledge. Stronger than ~だろう ('probably') and かもしれない ('might'). 訪問する ('visit, call upon') is a slightly formal verb for visiting; in everyday speech 行く ('go') is more common. イギリス ('England, the UK') is the everyday loanword (from Portuguese 'Inglês'), distinct from the more formal 英国. 訪問する takes を for the place visited — the place is treated grammatically as the direct object, not a destination with に.