I'e always felt that any 'terrain or climate' dependent advantages should be game specific.
That is, if in your game England starts on the coast, they should be eligible for a Sea/Naval Unique. IT SHOULD NOT BE A CERTAINTY without some other actions on England/the Gamer's part, though. Note that in game terms, "England" sat on their island from 4000 BCE to about 900 CE without any particular naval expertise or unique abilities. Not exactly a good basis for a continuous, starting Naval Unique.
There could/should be a bunch of peculiarly 'English" Naval Uniques, ranging from the current Royal Naval Dockyards to Sea Dogs to Battle Cruisers and other English-Unique ships and naval opportunities, but an England that starts in the middle of the desert will have no use for them, and shouldn't be saddled with them to the exclusion of more useful capabilities IN THAT PARTICULAR GAME.
And the game should have an option for Historical Start - not necessarily on a copy of the 'real world', but starting the Civ in something approaching to Real World conditions. In the case of England, on the coast, on an island just off the continent, possibly with another Civ on the same island (Scotland, Ireland, Wales, depending on the size of the map, of course). This would be dicey for the many, many Civs that are not in their 'starting positions' in 4000 BCE or didn't exist in any recognizable form until long after: even the Celts are about 3000 years in the future, while Greeks, Romans, Persians, Turks, All the Slavs (and England, of course, which wasn't even occupied by a recognizable 'English" population until sometime after several post-4000 BCE migrations) are nowhere near where they wound up when they first became recognizable: without some kind of 'mobile start' their Starting Positions would have to reflect their later, historical (or more accurately, 'Popular History") identity rather than their actual place in 4000 BCE.