A sketchead is British slang for a douche-bag. And the stereotypical douchebag thing to do is to turn up to parties uninvited.

Also a song by Arctic Monkeys, from the album "Cornerstone"
"David is such a sketchead, doesn't he realize we never invite him to anything for a reason?"
by _stx March 19, 2018
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Also less commonly known as a “bisexual abomination” on the social media website Tumblr
That Will, he must be a professional sketchead with trousers as fantastic as those!
by Dantanian Yadathan February 2, 2022
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