https://www.tuc.org.uk/join-a-union
Based on your industry it will show you the unions that you're eligible for, with quick summaries of each union and a link to their site so you can find out more.
It's a really useful tool for anyone who's thinking this union business sounds interesting but hasn't a clue where to get started… which, given the climate in the UK at the moment, is probably a fair few of us.
Fees for most unions start at around ~£8. Some vary in price depending on how much you earn or whether you're full or part time. Some unions also have even cheaper brackets for those in apprenticeships and those who are unemployed.