EARLY COSPLAY: 1960s
At the 1960 Eastercon (UK National Convention), held that year in London,
preparation had clearly gone into some costumes while other were pretty
ad hoc. Importantly, since everyone is on a stage in this photo this
would appear to be the first non-Worldcon UK convention at which the Fancy
Dress was an actual event rather than a part of a Liverpool Group party:
Then there was the 1960 Worldcon, photos by James O'Meara from Earl Kemp albums :
At the 1961 Eastercon there was a joust, hence the medieval costumes, that involved enthusiastic swordfights using wooden swords but with no quarter given. Surprisingly, few serious injuries resulted from this:
At the 1963 Eastercon - held that year in Peterborough - they decided to take the fancy dress out onto the streets to the evident bemusement of the locals. That's Charles Partington in the striped costume, others unknown. All photos are by Terry Jeeves:
In 1965 the Worldcon came to London for the second time. Photos by Peter Mabey:
Here's a link to a gallery of more Masquerade photos on the part of this site devoted to the 1965 Worldcon:
This is a friend in costume as Avluela from Robert Silverberg's NIGHTWINGS, specifically the image of Avluela on the cover of the contemporary UK edition, also shown. And, yes, the photo is used here with my friend's permission though without her name attached because this is the Internet. Although the photo doesn't show this, the wings were as long as those depicted on the cover, which meant a couple of attendants were required to support them. I really should have taken a longer shot. I'm not sure how much earlier you could have got away with such a costume - or if you'd want to chance it at some much more recent conventions given reports of the harassment female cosplayers have been encountering.
The second image, at the link below, was taken at the 1977 Worldcon Masquerade and depicts the 'Slave Boys of Gor'. Done with obvious political/satirical intent it features Avedon Carol with Steve Johnson and Ctein at her feet as the slave boys. Costuming done to critique particular works of SF was pretty new back then. However, the satiric intent may have been lost on several guys who sidled up to Avedon after the Masquerade, hinting at their interest in S&M. So it goes.