A witness to an electronic signature can validly attest to it by adding their signature, electronic or otherwise, when the party inserts their electronic signature in the document. The current view of how this can validly occur is that a witness must be physically present: they may not, for example, use videoconferencing to remotely observe a party’s signature being added to a deed.
It is likely that where there is a legal challenge to an electronic signature on a document, it would be accepted by a court as authentic unless evidence were given to the contrary. This is the same principle that would apply to a wet-ink signature whose authenticity is challenged.
There are various cases where electronic signing may not be effective and only a wet ink signature will suffice to make the document legally valid, binding, and effective. For example, certain documents that are required to be filed with the Land Registry need wet ink signatures, as do certain documents where tax, stamp duty or cross border issues arise. There may also be other exceptions.