The Association of British Science Writers (UK) – has an electronic mailing list and member directory as well as running events and advertising opportunities. There is a student membership rate available.
BIG STEM Communicators Network (UK) – a skills-sharing network focusing on informal and interactive learning.. Has an electronic mailing list as well as careers information, runs training and puts on events (including the Little and BIG Events).
A list of UK based learned societies and professional organisations – compiled by Michelle Brook. There is also a map version by Guy Fletcher-Wood (though there are some errors).
The National Union of Journalists (UK) – supports journalists of all types, including students.
Voice of Young Science (international, but based in the UK) – a network for early-career researchers to join. “VoYS members tackle misconceptions, challenge pseudoscientific product claims and respond to misinformation in all kinds of media.” There is also VoYS USA.