I do appreciate the amount of historical flavour that's been put into England/GB, and it's a fun country to roleplay, but it's not exactly top-tier in terms of special opportunities and so on. Particularly galling for lovers of English/British history is the way Mandate of Heaven turned any sort of parliamentary/constitutional option for monarchies into garbage once the Age of Absolutism comes in (not that parliaments are great even before then, given you have to sacrifice the nobility estate). Even if your plan is to conquer India, you're better off as a boring absolutist in the continental style.
Even if your goal is specifically an Anglo-French empire rather than an overall powerhouse, I'm not sure if it's better to force a PU on France as England, or to just outright conquer England as France. The diplomatic advantage of starting as France is that you're conquering 'outwards': because of its peripheral location, the only people who accrue AE against you when you take land in Great Britain are the Brits themselves, the Irish and the French. You are France, you're killing England, Ireland is a diplomatically irrelevant backwater and Scotland is also too weak to matter (plus conquering Scotland and Ireland is something to do while you wait for truce timers with England), so it's basically a conquest without any diplomatic repercussions: you can happily chomp away on the far side of La Manche while maintaining good relations with most of the HRE, Iberia, and so on. Contrast with forcing the PU on France as England, which feels good only until you discover just how many countries get pissed off by a major power shift in the French region.