This is probably all covered much better than I'm going to in previous threads, but I've found the following are key to beating that scenario:
***Note: SPOILERS AHEAD***
1) Don't attempt an immediate landing; dispatch the bulk of the Royal Navy first.
Trying an immediate landing brings you right in the detection zone of those nifty little RADAR installations that picket the landing zones...and this will bring the Royal Navy, Airforce, and odd ground unit swarming on your transports. It's important to stay out of the detection range of both the RADAR stations (which have a spotting range of 4 hexes, IIRC) and the Royal Warships.
To get rid of the bulk of the Warships, I've found having 2 Level Bombers and 1 Tactical Bomber works wonders, in addition to the two Level Bombers that you automatically get as auxillaries. Use the Level Bombers to take care of the heavy hitters (Cruisers and Battleships), and send the Tac Bomber after the English Sub. Line up your transports so that they're tucked in safely away from harm, and again, out of the range of the RADAR.
I personally don't ever use the deployment hexes near Dunkirk; they're too close to the Cruisers that come sailing out of the North for my comfort.
***SPOILER ALERT*** (Highlight to read the spoiler)
Destroy the Cruiser that's lurking in grid square (24,13) first, two hexes east of the port of Bexill On Sea. It'll wreak havoc on your transports until it's destroyed. I personally dispatch my most experienced Level Bomber, and shell it with the German Light Cruiser/Battleship until it's sunk.
***End Spoiler***
It's not uncommon for me to begin my landings by the fourth or fifth turn. Some of the transports will still have taken some hits from Bombers, but with luck, you'll get your whole fleet ashore without major losses.
2) Don't Send In Your Paratroops immediately.
I typically send them away from the English Coast (I send them to the easternmost German airfield near Dunkirk, still in their transports) until I've managed to damage the Royal Airforce badly, and then sneak them up to land around Ipswitch. Usually, I manage to keep the 1st German Heavy Cruiser alive, and it can aid with suppressing all the targets near the coast. That two-unit Paratroop team can then grab Ipswitch, Maldon, and the nearby Airfield without too much trouble...and that's one more objective out of the way.
3) Don't bog down all your forces at taking London, and don't overextend yourself in the city.
Once you've got your landing organized, don't plan on taking London first, and then working on the rest of the map, although it's tempting to do so. A sizeable battlegroup, heavy on infantry and artillery, should be dedicated to taking London, but at the same time, send a somewhat smaller team westward immediately to start grabbing Portsmouth, Southampton and Reading. Your surviving Cruisers and Battleship can help you to some degree with suppressing the cities along the coast, so the force doesn't have to be too large. Once you grab those three cities, you can send that battlegroup further west to take Bristol. The forces that attack London should still have enough time to head Northwest to tackle Oxford.
As far as London goes, the key here is to never put a unit where it's under attack (or potentially under attack) from multiple sides. It's better to destroy a force on a Victory Hex but not take the hex
immediately than to put your neck in the noose, and have the unit wiped out. Play it safe. I almost always try to advance my units two hexes at a time, and back both up with artillery.
I hope this helps...again, you'll probably find much more detailed explanations of tactics in previous threads, but this has worked for me a few times. I was expecting the good old "Panzer General" style unopposed landing the first time I played the scenario, and got badly mauled by the Navy for being presumptuous.
Good luck!