Why do the British pronounce the "R" when there isn't an "R", and they don't pronounce it when there is?

e.g.
Nassau (or as the british say, NassauR)
Idea (or as the british say, EY-DEE-ER)
Unknown (or as the british say, UN-NERN)


e.g.
Charlie is pronounced CHAW-LEE
Water is pronouncer WUH-TA
Teacher is pronounced TEE-CHA
"here" is pronounced "HEH"
Mother "MU-THA"
Daughter "DOW-TAH"

British english is fucking stupid