I gather the answer is that often they didn't.
It's also very easy to find coastal airfields. Just fly along the coast.
But those days were different. You could fing a feature on the coast and then zoom off on a heading. There was no CAS; I am sure a military pilot could fly freely.
Also, there is little or no terrain in the south of England. You could descend to say 500ft, and fly around at random, and the chance of hitting anything is very small (obviously not around Shoreham etc so make that 1000ft around there). The typical warm front cloudbase is 500-600ft, no lower. At 2000ft there is nothing to hit for hundreds of miles, unless you get incredibly unlucky. I guess they would make some effort to descend over the sea - much as many people do today.
If those battles had been done over Switzerland, I don't see how they would have got back down without navaids, radar or DF.
Today's cross country nav is a lot more regimented (regardless of what method you use) and if you get it wrong, a lot of **** hits the fan.