According to Kew Gardens, ancient Scots culinary herbal use was similar to that of the Scandinavians. Mediterranean cooking herbs were introduced around the 12th century AD. Before that time period, the Scottish highlanders were using local herbs and plants to flavor their foods and make jam or jelly. Here are a few:

Baldmoney (Meum anthamanticum) was used as a cooking spice.
Bilberry has plenty of vitamin C, and was used to make pies and to dye wool.
Bogbean was used to flavor beer, and as a tonic.
Cloudberry and European cranberry were used to flavor drinks and also were made into jam.
Oyster plant was used in salads.
Roseroot (Rhodiola rosea) a Scots herb, was also valued by the Greeks and Arabs.
Sea holly was candied.
Vetch root was roasted for its mild licorice flavor, and used, like juniper berries, to flavor food and drinks.
Wild garlic and wild marjoram were used to flavor foods.
Wild honeysuckle was enjoyed for the sweet juice in summer.

Archeologists excavating a Mesolithic site on the Scots island of Coll found evidence that yellow water lily and dock were boiled and eaten there.

What sort of food did ancient Scots season with their herbs? Evidence from the Orkney Island site of Skara Brae (3180-2500 BC) shows that ancient Scots raised cattle, sheep and barley. They also fished, using limpets for bait. Whale teeth and walrus ivory were used to make pins and other objects, but we don't know whether the Scots ate marine mammals. The Scottish dislike of pork and eels may well date back to prehistoric times, so these things would not have been on the menu.
So, in prehistoric Scotland, you could expect plenty of mutton, beef, lamb, and perhaps salmon, with barley beer to wash it down. There would have been salads of local herbs, too.