Well, it's not a lie that it's in my contract and that I signed it.
It is true that this clause is there in your contract, sure. But the clause itself is a lie - it’s void, because it contradicts the law, yet it is there, to make an impression that it’s not void.
There is no specific protection for it, but at the same time it also cannot be forbidden to be discussed. Employment protection in the UK is string enough that you cannot just fire someone for discussing it.