There is no single modern language called 'Celtic'; the languages classified as 'Celtic' are:
Irish, Manx and Scottish Gaelic as well as Welsh, Breton and Cornish. These are 6 separate languages.
The Celtic is a sea which is why, when you look in an atlas, it says Celtic Sea.
Celtic Sea and Mediterranean Ocean
In the sea view room you have glimpses of the ocean/sea, in the ocean view room you have the whole ocean/sea view from left to right.
North Sea, Irish Sea, Celtic Sea.
North Sea, Atlantic Ocean, Irish Sea, Celtic Sea and English Channel.
The Atlantic Ocean is the closest - it includes the Celtic Sea and the North Sea, between which is the English Channel. Stonehenge is between the English Channel and the Bristol Channel, another arm of the Celtic Sea.
The Atlantic Ocean, the Irish Sea and the Celtic Sea.
The Celtic Sea, which is really part of the Atlantic Ocean.
North Sea, Irish Sea, Celtic Sea, English Channel, Atlantic Ocean.
North Sea, Atlantic Ocean, Irish Sea, Celtic Sea and English Channel.
The North Sea, the English Channel, The Irish Sea, the Celtic Sea and the Atlantic Ocean.
Predominantly the islands referred to as the British Isles are in the north of the Atlantic Ocean, so that would be the single answer to the question. There are then various seas that are referred to, like the Irish Sea, the English Channel, the North Sea, the Celtic Sea and others that are at various points around or between the islands.