The last time a pair of Lancasters were seen in formation would surely have been for the Dambusters film in 1954?
Think it would be a bit later if you expand beyond the UK, as some were still serving until the early 1960s
Last 4 French birds in New Caledonia were withdrawn July 1964.
Canadian @1964
Argentine @1965
I'm sure there were a few formation flights with these Lancasters, but get your point- it has been a while.