WI: Brittany is separated from France in 1848

Prior to the Revolution, Brittany was not legally part of France, but considered a separate kingdom ruled in personal union. Say this arrangement is restored under the Congress of Vienna, and that following the February Revolution, Louis abdicates as King of France, but not of Brittany. This assumes that the Breton people still support him, as they did in that region's numerous royalist revolts during the revolutionary era. Is it realistic that the new French government lets them go, or would war and foreign support of Brittany (from Britain nervous about a new strong republican France?) be needed? And if Brittany does manage to split, what are the consequences? Does this weaken France enough to change the fate of the Roman Republic in 1849? Shift the balance of the Crimean War? Prevent the Entente between France and Britain after 1870, with Britain allying with Brittany instead? I think a lot of interesting things could happen here.
Brittany pilling a Taiwan probably requires a foreign power (Britain, maybe Russia or Prussia) backing them, which means you need a power willing and able to screw with France. Russia and Prussia strike me as the most plausible.
Maybe if Austria completely falls apart (no Russian intervention in Hungary?) and Germany ends up going republican Britain might be a bit more willing to upend the apple cart? Another possibility is if Belgium collapses into revolution or there's a major uprising in the Rhineland or both, and France starts meddling.
France would never accept it and Great Britain would have to keep an eye on Britanny at all times because at the slightest negligence France would jump on it.
While I'm not an expert in the period, I can't help thinking that it's a bit late. If you want to carve Brittany off after the initial inheritance, I'd use the Wars of Religion.
Unlikely to happen. France will not let Brittany go. Under no circumstances. The British cannot oust the French, the Prussians can't, nor the Russians or the Austrians. A coalition against France to save the sovereignty of Brittany is not gonna happen either.