Lundy, in the Bristol channel, has sika deer, but I don't think there are any reports of them swimming ashore.
However, a herd of reds were put onto one of the Tudwal islands (off Abersoch, Llyn peninsula) by Carla Lane (maybe 35 years ago?) and when they'd eaten everything on the island they swam ashore to Abersoch. They never established themselves in the area, but that is the reason why the BDS deer distribution maps show red deer on Pen Llyn.
(Incidentally, it's the same island that Bear Grylls is now the leaseholder of).