Slitherine

So I pretty decisively beat the first few missions, and I steamrolled the France campaign, but now I'm playing Sea Lion and the British fighters and heavy ships just completely destroy my forces before I can approach the mainland.

How do I actually survive until I can hit the British mainland?

Here's a thread that discusses some Sealion '40 strategies. I have a mockup map of my strategy; maybe that'll help you.

http://slitherine.com/forum/viewtopic.p ... ht=sealion

Air superiority is vital, fighters to clear out the British aircraft and level bombers to clear out the Royal Air Force.
Paratroopers are fairly important in this scenario, to seize inland objectives and secure airfields for your low fuel aircraft.

In addition to that, follow that thread, and you should be armed with everything you need to know.

Kerensky wrote:Air superiority is vital, fighters to clear out the British aircraft and level bombers to clear out the Royal Air Force.

You mean Royal Navy.

Yup, sorry referenced RAF twice there.
They really are that important to knock out, what can I say?