What is this young bird?

I spotted this young bird swimming close to the beach at Herne Bay in Kent. It looked a little lost and swam determinedly along the shore, just a few metres out. 

My 5 year old thought it was a penguin, naturally. Blush Google lens came up with some obviously dumb ideas but also suggested a young razorbill or guillemot which, from the online pictures, did look like possible contenders. Not birds I would expect to see in Kent though.

I'd be very grateful of some help identifying it.

Cheers

Richard