Bearing in mind that a ship's boarding party is normally under the command of a very junior officer (Midshipman, or Sub-Lieutenant) and very lightly armed I don't think fighting presented itself as an option.

During the Borneo Confrontation a 'Ton' Class Minesweeper, HMS Cheriton (IIRC) had a Sampan loaded with explosives go-up alongside during a board-and-search operation. Cheriton lost both of her Midshipmen that night - one blew his head off with a 2" Mortar illuminating the Sampan, then when the ship went alongside, the second Mid jumped down into the Sampan and set of a bobby-trap. After that, the lessons learned were for the mother ship to lay off one quarter, send one boat with the boarding party and another boat with a fire support group to position itself of the target vessel's other quarter. In that way all arcs of fire were covered and the minimum number of your own people were put at risk.

Can't think what the Commanding Officer of HMS Cornwall was up to, they obviously weren't keeping a very close eye on things, otherwise 'Cornwall' should always have been in position between the vessel being searched and the direction of greatest danger, i.e. the Iranian boats would have had to have passed HMS Cornwall to get to the boarding party. Irrespective of position 'Cornwall' should have challenged the Iranian boats on their approach.

No doubt more will come to light as the situation becomes clearer.