My understanding is it is a little more nuanced than that.
The truth is still an absolute defense against defamation in the UK, just like it is in the US, but the difference is who the burden of proof is on.
In the US, it is up to the person suing for defamation to prove the statements were false AND that the person knew them to be false when they said them (or should have known they were false).
In the UK, for someone to use the truth as an absolute defense, they have to prove the statements were true, which is often not very easy.
In both bases proof is only on balance of evidence, so the difference between proving something is or isn’t true is not as great as it would be if the standard was proof beyond reasonable doubt.
The truth is still an absolute defense against defamation in the UK, just like it is in the US, but the difference is who the burden of proof is on.
In the US, it is up to the person suing for defamation to prove the statements were false AND that the person knew them to be false when they said them (or should have known they were false).
In the UK, for someone to use the truth as an absolute defense, they have to prove the statements were true, which is often not very easy.