Being a Yank will have to excuse your mistake - Britain is made up of more than just England!!! Britain is an island - England most certainly isn't!!! In fact RIBs were probably invented at Atlantic college on the Welsh coast.
Britain has an incredibly long coastline for it's size - believe it or not actually longer than mainland USA!!! There is nowhere in Britain more than 70 miles from the sea.
As to sea conditions they can get pretty grim - especially on the West Coast. Britain is surrounded by very shallow water - that combined with Westerlies coming in straight off the Atlantic can produce some very nasty seas.
Also the Bristol Channel is cursed by some of the biggest tides in the world - the water goes out for miles in places.
I think it's down to all these reasons that RIBs are so popular here.
Having said all this there are also some very pretty calm stretches of water.
If you have a look at this you will see weather buoys you can get almost live data from - often you will see waveheights of 30 or 40' - mainly along the West Coast.
My local coastguard station reported a windspeed of 144mph a few years ago - we don't get much damage though as we get constant gales through the winter so trees etc only grow in sheltered spots.