Where are the roughest seas in england?

The roughest seas in the UK are those north of Scotland.

As a former ship's Officer who has served on various types of ship all around the United Kingdom coast, to answer this question factually it is first necessary to explain the difference between sea waves and swell waves and their effect on the ship and those on board.

Generally speaking on a large shi,p the most uncomfortable conditions to the guys on board are caused by very heavy swell waves coming in on the beam. As far as England is concerned the heaviest swell is likely to be encountered off the southwest of England either northwards or southwards of Lands End, most particularly off Devon or Cornwall or in or near the Bristol Channel.

The discomfort of swell waves striking your ship on the beam causes heavy rolling and is due to the length of the distance the swell waves have travelled across the Atlantic Ocean when they hit the ship, perhaps originating from a storm that may be hundreds or even thousands of miles away. Heavy swell hitting the ship on its bow or stern is more likely to cause damage, and lead to pitching and pounding instead of rolling but that does not cause so much discomfort to those on board.

Generally even though wind force on the North Sea.coast of England can be as high or higher than off the west coast, because the land mass of England will most likely shelter the ship from heavy swell, it will very rarely be so uncomfortable there, nor in the English Channel. As stated by another answerer it can be very rough too off west or north of Scottish coast, but the question specifies "roughest seas in England" not Scotland.

With a very small vessel like a yacht considerations are different. Because in that case the draught is very small, so any incoming swell waves will not affect the vessel like they do with a deep draught ship. So in case of small yachts or boats, it is sea waves not swell waves that is the consideration. Hence "rough seas" causing discomfort or danger to those on board are in their case equally likely anywhere on the English coast, when wind is force 9 or higher, and in those cases swell waves are not then relevant to discomfort nor damage potential of the craft.