I've had excellent results with a pair of BCs or BBs, a pair of CAs, and around 6 CLs per fleet (you want slightly more escorts than capital ships, in case you lose an escort or two). DDs are less likely to be hit, but CLs are more likely to pull back and survive if hit; you can take your pick, rather than having to research both. If you lay down the keel for your first BC at the start of the campaign, then add the second about 6 months later, that gives you plenty of time to research the current engine tech for your BBs, and then start construction on the first BB after completion of a modest round of IC construction. When the first BC completes, the increased Capital Ship Practical value will cause the second ship to move up about 6 months, and significantly speed up the BBs as well. The second BC will also significantly expedite construction the BBs. I generally have the 2 BCs, 2 BBs, 4-5 CAs, and around 8-10 CLs, along with a handful of long-range subs, either finished or just about to complete by the time I declare on Poland, not counting the pre-existing Baltic Fleet.
There's generally a CVL well under construction as well, the first lethargic CVL going with the obsolete Baltic fleet, and the newer, faster second CVL going with the BB fleet to provide air defense (it needs at least 2 engine tech upgrades to keep pace). By the time the first CVL completes, I've already laid the keel for the first REAL CV, which will have its own fleet, following the BC or BB fleets to provide air coverage. DO NOT mix full CVs in the same fleet with either BBs or BCs, because the CV will take priority, and all of your escorts will abandon the BBs/BCs to stay back with the carrier(s). The unprotected capital ships will then eat all of the inbound opposing firepower and die, rather than having 50-70% of that firepower directed at your escorts at poor odds to hit. CVLs don't have that issue, and can be added for air support, as long as they're fast enough not to slow down the whole fleet. Simple math: each of your escorts will draw roughly as much fire as each of the capital ships (doctrines will affect that somewhat), so 6 escorts guarding 4 BB/BC/CA means 60% of the shots will be directed at your escorts. If even half of the shots that WOULD have hit the capital ships miss the more agile escorts instead, you've reduced total damage suffered by 30%, increased the total amount of Defensiveness reducing the effect of those shots, and spread the remaining damage out among more ships. Once the escorts are knocked out of combat, the capital ships don't last long.
My surface navy parks in a Baltic port until France falls (no point in taking on the French navy if you don't have to), sortieing out only if the UK sends ships into the Baltic to harass my subs sinking convoys to and from Poland. After the fall of France (I hit the Maginot Line with Engineers and TAC bombers immediately, before the defenders get a dug-in bonus, leading to an early break-through), the fleets pull a Channel Dash to reach the annexed French ports below Brest, and heaven help the UK fleet that tries to intercept 2xBB, 2xBC, 1xCVL, 4xCA, and 12xCL protected by extensive land-based air support. Only then should you allow Italy to join the Axis, otherwise the combined French and UK navies will generally crush the Italian fleet before you're ready to assist. Once France is out of the picture, the German, Italian, and Japanese fleets should tear up the Royal Navy quite nicely. The old WWI BCs and obsolete DDs can stay in the Baltic for use during Seelow, and later to take on Soviet ships operating out of Leningrad.
Using subs as spotters and as bait (while raiding shipping off the north coast of Spain), I can usually pick on small isolated UK fleets, pouncing on them with my BCs and land-based NAV as they hunt my subs, then joining in with the BB fleet once they're pinned in place. If the UK carriers show up, break off before you take too much damage. By the end of 1940, I've generally sunk at least half of the UK's BBs for the cost of a CA and a couple of CLs, and the Japanese and Italians usually take out one or two more BBs.
When you've thinned the herd a bit, then you can pull off Seelow fairly easily. "Simply" (there's a misused word for you) move the old Baltic fleet to one end of the Channel, and your modern BB fleet to the opposite end, with the BC and CV fleets escorting the convoys in a sea zone between them. Use subs to provide early warning against anything major. The UK has to go through your other fleets before it can get to the transports, and the BC fleet can react to join battle at either end while the convoys unload. In one campaign, the UK put up a really tough fight at one end of the Channel; in another it couldn't muster more than a lone CL to attempt to stop me. Regardless, your BBs can sit and take a pounding for a long time before you need to break off and head for port, by which time your transports should have unloaded (air attacks will likely sink or severly damage several, so have a couple extra to spread out damage and/or cover losses). Once your troops grab a decent port to allow for the rapid transfer of troops and supplies without sitting offshore to unload (you can also deploy a pre-built level 1 port, but be aware that it will take time to become fully effective, and close to forever to upgrade), the occupation of England should be a relative cakewalk. I've taken out the UK by mid 1940, but in subsequent games I don't allow myself to invade the UK until I've sealed off both ends of the Mediterranean and effectively eliminated the Royal Navy's ability to stop me from crossing the Channel at will.
The Soviets get a random chance to mobilize and then declare war if/when you take any one of several major UK ports or cities, so have the majority of your ground forces ready on the Soviet border before making your landing attempt. Enjoy your world domination.
