Of course, an interest in older music, nostalgia, these things per se are not really retro as I'd define it. Retro is when a current group spurns the contemporary and makes music that is a remake or a form of attempted time travel. So that would be "Back in the USSR", Reuben & the Jets, and other things of that time like The Move's "Fire Brigade" of February 1968 with its discernible Eddie Cochran flavor (possibly the first retro move made by a major group?).
And probably not too long after that you would get the first of the groups whose wholesale music identity is revivalist - for instance, Shakin' Stevens and the Sunsets formed in 1969.
(Although looking into the odd story of Shakin' Stevens, it seems he never "went back" to rock'n'roll - that was always his only love, he was a fanboy of a 1950s Welsh rock'n'roller called Rockin' Louie! Another tidbit - Shakey's manager was a member of the Communist Party and Shakin and the Sunsets played a fair few benefits for the CP of GB)