France start.

I'm playing on H/H as France, for the fun of it.... and have been experimenting with openings.

So here is one:

First, get alliances with Württemberg and Westphalia, then your east flank should be secure.

Then you immediately declare war on Britain.... which will drag the UP into the war through the alliance between them.

Now you take your 5. rate sitting in the channel, sail up and engage the fleet the UK have in the Irish Sea. Remember to shoot sails, we need the 2 5. rates alive.

Once you've captured the 2 British 5. rates, you use one of them to sail all the way down and engage the UK squadron in the channel.... again, kill the sails, cause we need the 5. rate from this fleet as well.

Now, use said 5. rate and sail over and engage the Dutch fleet..... again, we want the 5. rate.

Now you've effectively cleared all UK and Dutch fleets from Europe.... and now you simply block every port in the UK and the Dutch port. Remember to place a ship on the link between Scotland and Ireland, so the UK can not get the army over.

Then you move your entire army towards the channel.... ferry it over and take London (turn 2 or 3)...

To be able to take London the same turn you land troops, you need to land in a harbour, so you need a harbour without UK troops blocking it. Which means you can not sail a ship into the harbour you want to use for the invasion and let it sit there (the AI will send troops to push it out, effectively blocking it for your invasion). So sail a ship in, and out again, just to "ruin it", so it can not produce ships.

This is about as far as I've gotten for a pretty good start you can always pull off.