I just played an England => GB => HRE game, mostly on 1.5. Here are some ideas.
There are plenty of guides on winning the 100 years war, look em up. Broadly, the idea is to RM and Ally Castille & Austria, pull them into a war with an Irish Minor, then pull them into the war with France. They should do most of the heavy lifting destroying France's army, then you can carpet siege and enforce a PU.
After that, consolidate your position in the British Isles, and look to Burgundy. They are your next natural target, they hold a core of yours, have territory you can sell to France, and and will want to try and take Calais from you at some point anyway. Initial wars can be tricky, since they will tend to have good leaders and a decent quality army. The trick is working well with France's army, at times it can just be easier to attach some of your forces to France's doomstack. The AI is good at pinning forces by countermarching, and you can use this to get a good engagement and defeat their main army. Over a few wars, you should try to conquer most of the coast at least, as well as some interior provinces to set yourself up wars in the HRE. Also, you should be insanely rich, so don't be afraid to use merc heavy armies, as there is a good chance your manpower will be low through most of this period.
Protestant or Catholic can both be powerful. France + GB will dominate the Curia if you're willing to put up with the minigame, but Protestant is a bit more inherently powerful, and can give you CB on most of the HRE if you take religious (which you probably will need to do to convert everything). Either way, wait till the reformation has calmed down a bit before integrating France. This will also give France time to annex more of their Vassals, which will make your life easier.
Back to the HRE. Try to limit for now the number of Burgundy's HRE provinces you take, they will be a pain, and can be free imperial authority later once you've become emperor. Speaking of that, even before you annex France you will be far too large to be invited to the HRE, so you need to get made emperor first. You can either try to vassalize electors, or win them over diplomatically. The former is easier as a protestant (free holy war CB, and there are 4 electors near eastern border of Burgundy), the latter as Catholic (since the HRE will tend to stay mostly Catholic most games). Once you're in, add your capital to the empire (or maybe consider moving it to the Antwerpen trade node if you've taken out most of your competition there) and you should have the emperor-ship in a lock.
If your goal is to form the HRE, the most important thing is to keep an eye on the big powers that are different religion. You can usually get small powers who are different, or large powers who are the same religion and your ally to agree to final reforms, but a large heretic Bohemia, Austria, or Brandenburg will probably never agree to Revoke. Try to get CB's on them, and get them to release countries. Ideally, you'll be working on this even before you become emperor, though it does become easier once you are.
Push the initial reforms slowly, and try to get most of your IA from conversions, forcing countries to be released, and returning territory. Try to limit the the number of your provinces you add to the HRE, they are a giant pile of free IA, and you'll be kicking yourself if you add them now but lose that IA when a OPM converts to a heretic religion. There are two big milestones for IA - Ewiger Landfriede and Revoke the Privilegia. The former bans war in the HRE, which limits your ability to break up large countries and force conversions, while the latter will kick out everyone who doesn't vote for it. Don't try to pass Ewiger till the HRE is a bunch of small to mid sized countries, and don't revoke till you can get almost everyone to agree. Most reforms can be passed with 60-70 IA, but with Revoke it's worth going to 100 to get as many as possible on board. This is also much easier if you do your best to keep good relations with the large powers in the HRE. Imperial authority by itself is usually enough to get small countries on board with reforms, but the large countries will only agree if they at least tolerate you.
At the same time, you should look for opportunities to expand outside of the HRE. One of the best options is to vassalize or conquer and release Aragon, then take all their cores back from Castille/Spain. Other options include southern Italy, Denmark, or the Baltic, though their viability will vary based on factors outside your control. The goal is to get as many provinces outside but adjacent to the HRE as possible, which is extra IA for the final push. You should also look for countries in the HRE with lots of cores who have taken a beating - you can vassalize them, fight wars to return cores, annex, and then release them in the final push. In my game i did this with Savoy, though depending on how your game plays out, Milan or Switzerland may also be good options. Annexing vassals in the HRE is risky, it will hurt you in the short term, but it can be a free store of IA for the final push.
Once you pass Ewiger, most of your IA will come from adding your provinces to the HRE, and releasing vassals. Make sure you've added all of a vassal's territory to the HRE before releasing them, as this will maximize your IA gain and make sure they are a member. Release small countries first, this will get your more votes, and let you pass reforms before Revoke with less IA. When you're doing this releasing vassals strategy, it is extremely important you pass Renovatio Imperii immediately after passing Revoke, as you will be a disjointed mess with far too many vassals till you do. Once you've Revoked, the last reform isn't that hard, and suddenly you'll own most of Western Europe.
From here the world is your oyster, you're spectacularly rich, drowning in Manpower, and in the best tech group. Early on, focus on converting provinces and conquering anyone who didn't agree to join the HRE - those claims only last 25 years, so get to it. From there on, pick grand challenges, and do your best to accomplish them. In my game, i vassalized and annexed Naples, kicked the Ottomans out of Europe, got the Mamaleuks released, helped them take most of Arabia from the Ottomans, and even vassalized and annexed Granada when they revolted from Spain. That's fairly minor, there's really nothing you can't do as the HRE formed by an England who won the Hundred Years War, though 10+ years coring times make expanding by vassals basically your only viable strategy.
Have fun, and i hope this helped.
