Page "Loch" Paragraph 9

English borrowed the word separately from a number of loughs in Northumbria and Cumbria.

Earlier forms of English included the sound as gh ( compare Scots bricht with English bright ).

This form was therefore used when the English settled Ireland.

However, by the time Scotland and England joined under a single parliament, English had lost the sound, so the Scots convention of using CH remained, hence the modern Scottish English loch.