Although the nobility of Wales imported wine from the Roman period, and perhaps before, mead was the primary drink served throughout the country for thousands of years.  Because of the climate, grapes, many fruits, and even grains at times do not grow well in Wales, though wine production did (and does still) exist: “Wine has been made in England and Wales since Roman times. By the time of the Norman Conquest vines were grown in a number of vineyards, many of which were attached to monasteries. In fact the Domesday Book (1085-1086) records vineyards in 42 places. The main areas of production at this time were the coastal areas of the southeast, Somerset, Gloucestershire, Herefordshire and Worcestershire. From the Middle Ages to the 20th century there was a decline in vineyards and the reasons cited for this have varied. They range from the Black Death that caused the depletion of labour and lead to many landowners renting out land rather than working it themselves, the breaking up of the monasteries in 1536, change in climate and increased volume and quality of wine imports from France.”  http://www.defra.gov.uk/foodfarm/food/industry/sectors/alcohol/wine/industry.htm

Mead,  however, was a local product, made in Wales as well as in native cultures throughout the world.  “The first meads were most likely made simply by taking honey and water and letting them ferment with the naturally occurring yeasts found in the honey. Evidence of early meads has been found in Egypt and on the island of Crete, and it was drunk in Greece throughout the Golden Age. In many early cultures, bee goddesses held central roles in the pantheon, and many have postulated that this was because of the intoxicating effects of mead harvested from local bee hives.”  http://www.wisegeek.com/what-is-mead.htm

“Despite what most people think, mead is not a kind of beer, as the definition of a beer is an alcoholic beverage made from grains. In Europe beers were made from barley and wheat. In Japan beer is made from rice (this beer is called Sake) and for the ancients of Mesoamerica beer was made from Maize (Corn). Mead is made from water, honey and yeast; as such it is not a beer. Neither is it a ‘wine’ as the sugars involved in fermentation are not derived from fruit.

Mead is mead, an ancient drink much beloved of the Celts and the peoples of Europe during the Middle Ages. For mead brewing, the initial mixture of water, honey and yeast is termed a must and the yeast converts the sugars in honey into alcohol at which point the must becomes mead. It is possible to create different flavors by adding ingredients such as fruit or spices into the Must, or by putting them into the Mead when Fermentation has stopped.”

Indications that mead was drunk in Wales, along with wine, is found in the Y Gododdin, a 6th century poem by the Welsh poet, Aneiron.   The poem tells of the ill-fated soldiers who were selected by Mynyddog Mwynfawr, the ruler of the Gododdin, for the battle.  While they prepare, Mynyddog housed and feasted the men with food and mead.  In addition, there are also other references in early historic poetry to ‘talu medd’ – payment of mead, in which soldiers became obliged to fight for the leader of the battle in order to repay his hospitality.  http://www.bbc.co.uk/wales/arts/sites/early-welsh-literature/pages/aneirin.shtml

Although I don’t drink myself, here’s a relatively medieval recipe for mead: https://digventures.com/2015/04/tasty-medieval-tipples-make-your-own-mead/