I also saw one from the train going over Kew Bridge. I thought it was a person and couldn’t understand why they were swimming in the Thames on a cold day. Then realised it was a seal. Quite cheering really!
Close to Richmond. I thought it was a dog at first - a dog-like head - but then it went under and we saw the back surface vey briefly. The head appeared again very briefly quite a bit further down. Apparently sightings are not uncommon. It was going the wrong way to get back to the sea, though. 🙁