In my recent experience with BA, the last flight needed to depart 'a day before' the first flight had been flown the previous year. I.e. the first flight was on 25 Oct 2016, BA would not allow the last segment to have departure date of 25 Oct 2017, it needed to be 24 Oct 2017.
I am currently on a rtw issued originally by BA in South Africa. Although throughout process of the original issuing and subsequent reissuing of the ticket I usually spoke on the SA telephone line, I was never sure of whom I was talking to or where he or she may have been physically located. I also used the Brazilian 0800 number, which often connects through to people in Portugal. In any case the first flight was on 27 April 2017 and the last flight is currently scheduled for 27 April 2018. The whole issue of 1 year or 365 day limit never came up.