If the terms are relatively lenient, you're less likely to the kind of crippling economic downturns and political strife that would give the radicals the leverage to take power. Maybe there's some instability/civil strife in France, but the government would retain enough power and legitimacy to keep a hand on the reigns and get the nation through the worst years. Long term, I think France is going to end up falling into a malaise; the lose in The Great War would demonstrate that, even with ideal geopolitical circumstances and all the Élan they could muster, France is unable to crack Germany and is out of options to compete politically, militarily, or economically with the new giant. Perhaps they take a post-WW 2 UK approach and try to play Greece to Germany's Rome; making a big push to establish French cultural supremacy (perhaps adopting a light socialist approach and encouraging domestic liberal ascendency in Germany) in Western Europe. Perhaps the demographic collapse causes the Empire to slowly crumble as she's unable to maintain an army and thus lose preferred access to markets/raw materials, leading to the nation slowly turning into a modern Medditeranian debt-state over the course of a few decades. But i highly doubt you'd see the Fascists take power.
As for Britain, it's a lot more varied. Ireland is probably going to see a partition similar to OTL: the Home Rule Bill is already on the books, though there's a distinct possibility the Irish Parlament in Dublin remains merely a devolved state within the Empire rather than making a push for full independence if there's a "Cold War Climate" between them and Germany (and cooler relations with the US; without belligerency on the later's part or German reparations to funnel into a repayment scheme Britain is going to be stuck holding a lot of debts that could very well serve as a sore spot between them and the American business community... which would remind them of the commerce they'd already been 'cheated' out of by the Blockade and could face in the future from protectionist policies put in place on the Imperial markets; both as a revenue measure to pay for war bonds and debts as well as secure British industry from increasing German penetration) in which Ireland might become aknife pointed at her hip. They are, in my opinion, far more likely to take a right-wing turn than France; unlike the later her forces preformed by an large admirably during the War, she bankrolled the Entente who not only failed to carry their weight and betrayed her by pursuing separate peaches/abandoning her, has the resources to fuel a national resurgence and pursue a autarkic policies, ect. Ideologically this probably looks like Edwardian/early Victorian condescension, protectionism, Imperial unity "One King, One Flag, One Fleet, One Empire", and perhaps some elements of WASP racism (they may be rivals with the Germans and Americans in a cultural sense, but they fact that those two nations were the ones challenging British dominance would be cast as proof of the race's superiority). Britain would distinguish it's higher destiny/position over its racial brother-nations as a matter of culture; saying Germany was too close to their barbaric roots while the Americans had strayed too far away, Britain's moderate dilution of the Anglo-Saxon character with just enough Celtic influence to dull the sharpest edges being the ideal.