I don't think I've seen anything discussed like this before. The search function is being silly so I apologize if it's been posed before. But anyways, the main question concerns what could be an "Unholy Alliance" in the 18th century, it's plausibility, and it's effects on the politics of Europe.
For the 18th century (or perhaps since 1688, more accurately), Britain and France were locked in a deadly struggle, knocking heads several times over the course of the century, not just in the old world, but over in the colonies as well. Britain was quite keen at forming continental alliances to botch French expansionism and to prevent a Bourbon hegemony on the continent. But this wasn't always so: prior to the Glorious Revolution, France and Britain hadn't been on awful terms. It probably didn't hurt that Charles II was eating out of Louis XIV's hand, accepting subsidies so that he didn't have to call parliament, and that they had a common foe in the Netherlands. The rivalry between the two nations is historic, yes (reaching back to the Hundred Years War, and even further back), but there were also cases of them being friendly (Elizabeth wanting to aid Henri IV).
So WI: instead of spending the 18th century duking it out, France and Britain become allies? Obviously the Glorious Revolution has to be prevented; it was William of Orange's francophobia, and Louis XIV's desire to have his cake and eat it too which were the biggest causes for the War of Spanish Succession. If 1688 is prevented, perhaps James II is less antagonistic in favoring Catholics and isn't so headstrong in creating religious tolerance, he can maintain the throne, albeit uneasily. No doubt Louis XIV would look at this favorably and would probably fund James as he did Charles II. When the Spanish King croaks in 1700, no doubt Louis XIV will still move to claim the inheritance for the Duc d'Anjou. However, there were several things which upset the English and the Dutch and drove them to aid the Austrian Habsburgs: The French occupying border forts in the Spanish Netherlands, and closing the Spanish Americas to English and Dutch trade.
If Louis XIV does not carry out these measures, the inheritance is no doubt secured (at least on paper) for his grandson. France will still have to fight the Habsburgs, and William of Orange (just Stadholder of the Netherlands in ATL) will probably jump to their aid -- I'm sure other German states will follow and maybe even some in Italy, but England will probably remain neutral, if Louis XIV throws them a bone or two to keep it that way.
That is just the basic idea. What effect would an Anglo-French relationship, evolving from that which existed since the English Restoration in 1660, have on the politics of Europe? What would be the required POD(s) to make it happen? Would the British simply have some form of splendid isolation, tactically approving of the French actions on the continent assuming they do not get too out of hand, or might an active alliance be a possibility too?
There are also plenty of butterflies, such as the Bourbons gaining the whole of the Spanish inheritance, no Utrecht, a wholly different War of the Spanish Succession.. not to mention conflicts down the road; will a proactive France seek further influence in Germany and Italy as they did IOTL? In the east, such as their adventures in the 1730s to support Stanislaw to regain the Polish throne?
These are just some rough ideas. I know that
Endymion's French Hegemony TL features a powerful France and a friendly England tied to them, but I was considering another direction. With my mind currently stalled with my Miguel da Paz timeline, I am mulling over a possible TL, less text heavy involving this possible idea.