As Merry says, ideology come second to power-politics as soon as you take over a country. Spain, after all, was neutral and Portugal, IIRC, British-inclined; whereas the whole period when Moscow-line communism was aiding and abetting fascists in Europe puts paid to the idea that the struggle was purely ideological.
If Britain were to be allied with Germany it would require different regimes, different circumstances, and a different balance of power.
1) If we suppose some sort of non-Hitlerite military-authoritarian-nationalist regime in Germany - call it fascist if you like, I think it would be more of a Poland than an Italy - then perhaps we get what we wanted under the appeasing governments: a broad revision of Versailles creating a strong Germany in the middle of Europe with land armed forces proportionate to its size and a normal economy, with some sort of stake in Africa to keep it aligned with the conservative Entente against the 'outside powers' (tut-tutting America, erratic Japan, Cossack Mongol Commie Horde USSR).
(I must stress, however, that the friendship of Germany would never be bought at the expense of France, as a certain sort of person is so keen on doing. Many in Britain regarded France an annoyingly belligerent, but we were annoyed at their belligerence precisely because there was no question that if it came to it we would back them.)
Under what circumstances a war would be fought is the question: Stalin took a few big risks in his time, but with his nightmare alliance materialised he will be on his tippy-toes. Of course, as Faeelin often points out, one can never discount the possibility of aggression from The Good Guys.
2) Let's go further back and have Germany win WW1 at some time in the middle of the war by handwave. Defeated France is neutered and with Italy becomes a medium-sized power in Germany's grossraum, its conservative and authoritarian government troubled by constant civil disturbance and political agitation.
Britain, having had to make some sort of compromise peace, slides into a clumsy half-hearted reactionism, keeping the lid on Ireland, India, and the Unwashed Masses by increasingly heavy-handed measures and floating unwillingly into the German orbit.
America affirms its commitment to China and the Russian Socialist Federation...