There are many solutions to your problems, e.g.:
a) guard each port with 2 garrison divs (no attachments and they are dirt cheap). Have a fleet (can simply be a dozen subs) nearby. If you park them in a port and set them on "naval scramble", they will automatically disrupt every invasion attempt in their sphere of influence. As UK I've sunk more than a hundred Japanese transport ships who were constantly trying to invade Hong Kong and Singapore. Never had to care about the invasions due to the naval scramble.
The next two will deal with Britain, so you won't have to worry about them in the first place:
b) Sealion. Instead of invading Norway, fighting in Northern Africa and the Middle East, just sealion them Brits. It's tough (especially since the Exeter exploit was removed), but doable. Controlling the Channel is easy with some dozen subs and 12 fighters. Then do several invasion attempts in different locations. The a.i. reacts to an invasion by shifting its troops to the hotspot. Attacking west means less troops east (as they move west). Keep the a.i. on its toes and you'll find a hole in its defences.
c) Indirect approach. Build ~150 subs, form fleets of 12-24 subs and set them on infinite, night-only convoy raiding in the Atlantic. It takes 2-3 years for the UK to crumble. Once she runs out of convoys, Africa and the other non-mainland Britain theatres resolve themselves. The mainland won't be able to sustain itself either and becomes easy prey.
For added effectiveness, I always grab Denmark in 1938. My U-Boats operate from Iceland then which is pretty sweet.
It's all about the numbers though. Subs in stacks of 12-24 are able to fend off any surface navy and will go rampant on them convoys.
I proposed a U-Boat nerf, so that tactic might not work as well anymore. Fernando will know more about that.
Personally I love the U-Boat strategy because for the first time in any HoI-game starving Britain actually works and it is so much fun! With the loss stats following the course of your U-Boat campaign is extremely exciting. It took me a couple of tries before I got the numbers right.
Yeah, as I said, I already have 2 or 3 INF div (sometimes with AA brig) in every provinces with beaches and still they manage to beat my defences. That's ok, I just need to reinforce the western front. I actually put all my effort on the east front (I was used to the previous WIF version.. so I expected a LOT of divisions on the border so I used my IC to build a lot of ARM and MOT).
My building schedule was
1033-1936
15 IC x 3
1936-1941 (gradually added, not all in the same time..i had not enough IC, I still needed reinforcement for west air attacks)
1x6HQ
1x12FIG
1x12INT
1x12TAC
1x12CAS
1x4NAV
1x4STR
1x3TRA (maybe some paratroop operation in ENG in later 42?)
--
6x6INF (aa-art-eng)
2x6ARM
1x6MOT
6x6?various SS
6x6SUB
2x1CV
2x1BB
1x1BC
1x1CA
3x1PAR
Maybe I'll write an AAR at the end