I think this is the Falkland Islands vessel DISCOVERY II of 1929, and it may well be her maiden voyage, calling at London from her builders in Port Glasgow. There is a pennant flying at her foremast with what could be her name on, though I cant quite make it out properly. It is possible there was some sort of official naming or commissioning ceremony performed there. I do recall seeing other photos of her in the Pool of London. The crows nest and searchlight on the foremast do set her apart from the run-of-the-mill cargo ship.
a smart bit of work there Graham , following up I found a similar photo as she passed Tower Bridge (image kindly donated by the Dundee Heritage Trust ) - confirming your observations;
" RRS Discovery II sails from St Katherines Dock to the Southern Ocean for the first time 14th December 1929"
"10 December 1929 at St Katherine's Dock, London the ship was inspected by the Duke of Gloucester"