Walrus are thought of as an arctic species, and indeed this is where they are found in their greatest numbers. However, they once had a greater range. Walrus have been extinct here for 3000 years, however the UK is part of their natural range, if an extreme southerly point.
As a result, there are a handful of occasions when they venture this far south, with a couple of sightings each decade. Whether continued global warming will put an end to this, or whether it will force walrus to swim further and therefore they will become more common is unclear.
These are totally natural sightings, this is not a zoo escapee. I would love to see walrus extinction reversed, the British coasts are well capable of supporting these animals. Whether that will ever happen is uncertain.