Distance is not always the problem. Cold water might be.
There is an island in the middle of that lake in Siberia, the one gulag escapees always have to skirt around. A former g/f told me she trained it like 3,000 miles to see, from Warsaw. It's famous because lots of escapees are Polish, stuck in some gulag in Siberia.
I'm not a good swimmer, so anything more than Alcatraz might be beyond my ability.
If you were stuck in England, you might not make it to the mainland.