So are you saying that if our nice builder receives a salary of £38,000 and a dividend of £10,000 he cannot choose to allocate £5,000 of his personal allowance against dividend income leaving £5,000 to be covered by dividend allowance, and the balance of £6,500 against his salary - a tax saving of £625 over the HMRC software calculation which would allocate all of the personal allowance against salary????

No, I am saying that ithout all the facts of this 'supposed' trader you cannot (reasonably) come to the conclusion you've given.
And the latest figures you've given do not match up with the earlier example.
A little consistency might help! Or have you given up on your earlier example, and are trying to give a new one (which is equally bereft of sufficient detail)