I just followed the official requirements list from SEF/AIMA, which has this point listed. So I cannot tell what would have happened if I did not supply that declaration. On the other hand, my lawyers did not flag this document as ‘unnecessary’ either.
From the two method I listed in the linked post, the first one is longer to set up but solves the problem permanently as in the end you’d have your own online access to this declaration any time. The second one is quicker short-term, but it requires repeated appointment either in person or via lawyer every single time in the future.