... for doing much more harm to the british isles as IOTL that could be rendered even more existence-threatening than IOTL as well, making an eventual - after another change of minds of the dictator ? - peace-proposal look even more interesting than IOTL.
Notwithstanding a still possible later switch-of-sides of the Wallies again, when Hitler eventually turns against the SU later.
Both is possible yes. A lot of people on this forum subscribe to a very rigid interpretation of Nazi/WW2 history. I am not a fan of this.
The whole trope that only "unconditional surrender and nothing else" was possible after Germany initiated WW2 is wrong in my book. Between the end of Fall Gelb and the failure of Barbarossa there is a big window where the war can develop in a lot of ways. Denying that ignores how dangerous the Nazis where, how much acceptance or even symphathy there was for Fascism and that war is more than a block of industrial statistics in a history book.
Britain in 1940 is still ruled by politicians who view it as an Empire. In 1940 it is still entirely possible to rationalize Nazi behaviour as "WW1: Season 2" (especially if the person in question has some symphaties for Fascism). From a certain imperialist point of view it makes a lot of sense to cut a deal with the Nazis. It was clear after the BEF barely escaped, that the British Empire as a first rate power would be finished if this war continues, even if Britain wins.
Adding 2-5 years (depending on how the alt-Western Front plays out) on top of that, where the Nazis are free to throw their whole warmaking and terror ability against the British Empire, makes a negotiated peace not that unlikely. Its not like the USA is able to protect the British Isles in 1940,1941,1942 or maybe even 1943.
Hitler, in his twisted worldview, actually intended to leave Britain as an overseas Empire. He wants the continent as a sphere of influence, so he would obviously ignore all the prior treaties trying to reign in his influence on the continent. This doesn't mean that Hitler would attack the British Empire once he has what he desires from Britain. I very much doubt that all British decision makers at the time believed that
every deal with Hitler was worthless.
And yeah, a switch-of-sides is still in the books. By 1940 the Soviet Union is still a pariah state that fueled the Nazi war machine for its own sinister goals.