The portrait shows someone with the Order of the Garter with the Great George (the image of George slaying the Drgaon), which facilitates the identification of the sitter, as this is a very exclusive club.
Some searching, helped by the fact that the sitter indeed was a Howard, led me to Charles Howard, 1st Earl of Nottingham (1536-1624), the English commander at the time of the Armada, so a major figure in English history of the period.
The painting for sale is a poor partial copy of a portrait by Daniel Mytens (the major portrait painter in England before the arrival of Anthiny Van Dyck), from ca. 1620. It is now housed in the Greenwich Museum. Some colours have been changed, so perhaps it is based on an engraving, but it seeems more likely to be a copy of the painting with some artistic license added.
The value of the work is probably about the estimate, but having the correct identification may help attract the bidders: I wiould guess there are more marine history buffs wanting this, than people interested in a portrait of some Cardinal Howard.