During a cruise up the Thames, we stopped just opposite Battersea park for some lunch, when someone said they saw dolphins. We all carefully watched the spot, and there they were again. Two small dorsal fins appeared briefly side by side, then disappeared again. They were swimming upstream against the tide but were quickly losing ground. We reported this to ZSL’s Marine team who came back saying they were most likely Harbour Porpoises. Heres the information they gave:

When feeding, they commonly surface to breathe about four times every 10 to 20 seconds before diving for up to six minutes, it is possible that this is the behaviour you saw. The European population has been declining significantly for the last 30 years so this information is of great value to us. The decline in their abundance is mostly due to bycatch in commercial fishing nets.
Generally these creatures are shy so congratulations for managing to spot one! Harbour porpoises feed mostly on fish and are the smallest of all the cetaceans (whales, dolphins and porpoises). A good way of distinguishing them from their dolphin relatives is to identify their blunt round head and small triangular dorsal fin, which you did.