Well it was U-29 which was sunk by a Dreadnought, but it had been U-9 which sunk the three cruisers. Of course, Weddigen had been commander of both.
Even if it had been defenseless and surfaced, Churchill's order was that "Shipwrecked crews [of U-boats] should be arrested or shot, whatever seems practical." A fleet being threatend by U-boats would have most probably made a ramming "more practical"