Actually its sinking in the east and rising in the west , as well a a slight south north tilt. As to how it happens, remember that the surface of the earth is a thin skin of solid over a liquid centre(think liquer chocolate) and is in constant motion. Consequently the odd wobble is to be expected.
It is due to isostatic readjustment. This is a process caused by the Ice layer on Britain during the Ice Age. the Ice melting meant that the surface of britain sprung back in the north, where the most ice had been.