Disciplīna in Britanniā reperta atque inde in Galliam trānslāta esse exīstimātur.
"The discipline/teaching is thought to have been discovered in Britain and carried over from there to Gaul."
The key here is that disciplina existimatur is passive: "The discipline is thought . . . ."
reperta atque . . . translata esse are perfect infinitives, but reperta and translata are nominative, not accusative, because they agree with the subject, disciplina.
He could have used an active verb: Disciplīnam in Britanniā repertam atque inde in Galliam trānslātam esse exīstimant. The subject of existimant would be indefinite, "one believes that" or "they believe that." Here we would have indirect discourse with an accusative subject and the perfect infinitives would be accusative. But when the passive is used, and the logical object (disciplinam) is grammatically the subject of the main verb and in the nominative case, the perfect infinitives agreeing with it must be nominative, too.
He could also have omitted esse, as Latin authors frequently do. But the underlying forms are perfect infinitives, which are formed periphrastically with esse + past perfect participle.
Hope this helps.