There are two great resources here:
The Activist Handbook communication toolkit which guides you through communicating in all kinds of media on all kinds of platforms. It also signposts to other resources for everything from ‘how to persuade people’ to ‘how to organise a press conference’. The Press Officer’s handbook from NEON, which has a wealth of useful information, including a list of all the main newspapers and radio stations in the UK, their readership/listenership numbers and their political leanings. It will walk you through things like how to write an eye-catching press release with clear step-by-step guidance.