Skomer Island and Puffins
Itinerary: Train from London to Milford Haven in West Wales (5 hours). Taxi from station to The Marloes (10 miles). Stay at the Marloes. Walk to Martins Haven and catch the 10am ferry to Skomer Island(15 minutes ). Full day on the island.. Ferry back home . Go to Little Haven and have dinner at St Brides Pub. Next morning enjoy the beach then train back to London 5 hours).
Take your lunch with you to the island, and you can rent cars instead at Haverfordwest where the train stops as well.