Some general thoughts:

1. I think finishing at the end of Turn 18 is easily possible. I basically took London and then was waiting for England's war support to reach zero. At one point it was going from 5, next turn to 1, next turn to 0. Hence, just by initiating a battle and a retreat in time, one could bring the 20 turns down by 2.
2. I would be very curious to see whether someone does it in 15 Turns or less, maybe without teching advanced military and just going Archers+Swords. Might be possible since the opponent is quite a push-over. On the other hand, if he manages to kill some of these squishy units with the new war support mechanic, he gets war support and you lose some. That might drag the war out and even take the chance from you to vassal. Curious to see.

Some specific tips:

1. (I think) main strategy is getting war support fast, draining war support of the English and of course, taking London. The main tools in your arsenal will be the Aesthete Trait. Use it to create loads of influence. With that you:
   1. claim territories, connect the necessary resources, create cities and make them big and productive by adding outposts (don't mind stability or building anything big; you only play for short term here anyways)
   2. "Influence-Scout" your Territories: Move the outpost around using influence like crazy to discover and explore the map. (In general, I think it is a good advise to start with one playthrough for exploring the map and making screenshots of important areas.)
   3. shift the culture of Englishs outpost on your coast to yours and demand it, influence-purchase a harbor at a good position, attach it to your best (most productive) city, build/buy a garrison next to the harbor and produce units there, let them sail to the English coast and set up your attack on London. Garrisons in general obviously, save some time.
   4. claim and build Forbidden City (that is key for the War Support); Lighthouse can also save a Turn since it gives you that little more movement to cross the channel one turn faster.
   5. always have the fitting civics enabled. Especially a Conscript switch into Professional Soldiers can come in handy at the right moment. Homeland civics should obviously be enacted as well for the combat strength.
2. I think you won't need more military-tech than Crossbows. Take some swords with them and that should be fine, I guess. I also did Longswords but I think I just wasted time by doing that. As I mentioned just going Archers+Sword might allow some very quick finishes (15 Turns and less) if done correctly.

Franks Turn 20.ctr