While truth is still a defense, in the UK the burden of proof is on the defendant; this means that if you are accused of libel or slander in the UK, you have to prove that what you said is true in order to defend yourself. This can be very difficult; for example, how would your PROVE a hotel clerk said certain things to you?

In the US, the burden of proof is on the person suing for libel; they have to prove that what was said is false.

So in the UK, yes truth is a defense... but you can still be telling the truth and unable to prove it, which makes it not much help.

https://en.wikipedia.org/wiki/English_defamation_law