Sites/apps like Twitter, MumsNet etc can be compelled by court order to disclose identifying information related to 'anonymous' accounts.
While using a VPN to create/post from a social media account adds a layer of difficulty to tracing it's origin, it absolutely does not make it impossible, or even difficult, for anyone with knowledge/resources. This is especially true when it comes to free/cheap commercial VPNs.
If you want to post a potentially libelous statement anonymously online, you should first seek expert advice around protecting your identity.
UK law is very strict when it comes to libel. People will tell you that you can post anything you like, as long as it's true. This is often extremely dangerous advice.
A statement being 'substantially true' is indeed a legal defense available to someone accused of libel, however it does not prevent legal action being taken. Defending a libel claim that actually makes it to court will cost 5-6 figures; most are settled - even by people who believe themselves to be in the right - on this basis alone.
Additionally - and unusually - the burden of proof in defamation cases is on the accused.
If the crown accuses you of - for example - burglary, they must prove your guilt. You are not required to prove your innocence. If I take you to court for making a defamatory statement about me online however, the burden is on you to prove the veracity of your claim to the required standard. In broad terms, you must evidence to a court that your claim is 'more likely than not' to be true.
As with anonymity, taking expert advice is essential before making potentially libelous statements in a public forum.
Speaking to a reputable journalist from a reputable media outlet is a good option, but again, you will need to evidence any potentially defamatory claims, even if you are to remain anonymous in any resulting article.