The shores and seas of Britain are kept warm by the influence of the Gulf stream. So we end up with the wet and mild climate that we have. Other areas of the European/Asian continental land mass and North American continental land mass which are at the same latitude or even closer to the equator have much harsher and colder winters than we have. The off shoot of this warmer climate and sea current influence is that fish (and reptile / mammal) species associated with (sub)tropical areas are often seen in our seas. Indeed in the early part of the last century, there was once a thriving North Sea Tuna commercial fishery operating out of Whitby North Yorkshire.
It is very easy to blame global warming on the effects of mans pollution of the planet etc., on that note how do the scientist explain the recent warming of the surface of the moon and our local planetary neighbours Mars and Venus. Current thinking is that the suns activities are causing the additional warming. During the Roman occupation of Britain, the climate was warm enough to make wine from grapes grown in vineyards located in the Vale of York.