Bismarck famously said he would call the police if the British ever landed an invasion force on German soil (referring to the small size of such a force and the inability of any British force trained and indoctrinated for colonial warfare to stand up to an equal or larger German continental force).
In 1915, that would have still been the case, perhaps even more so. Germany would have had to redeploy forces to contain and then destroy the landings but England would most likely have stripped forces away from the Western Front to support it too.
Alternatively, they could pull anther “Salonica” and allow the Allies to form yet another “largest voluntary prisoner of war camp”……