。
She'll definitely visit England this summer.
Literal
She [topic-は] this summer England [object-を] visit [must-be-にちがいない].
~に違いない ('must be, no doubt') here applies to a non-past clause — the speaker is highly confident she's going to do this, based on what they know. Japanese has no dedicated future tense, so the dictionary form 訪問する covers the future, with the time word この夏 ('this summer') anchoring it. 訪問する ('visit, call upon') is the formal verb for visiting and takes を for the place visited — the destination is grammatically the direct object. イギリス, the Portuguese-derived everyday term for the UK, contrasts with the more formal 英国. The whole sentence has a slightly written or news-flavored register.