Haven't you got a sort of anomalous climatic area in Scotland on the west coast somewhere? I vaguely remember reading something about the Gulf stream washing those shores and causing warm air etc and a wholly 'abnormal' for those latitudes, climate.
Ask most people in Britain and they will say, we don't have climate here we have weather.
It is unsettled and almost every day can be different including temperature. I am southern English now living in northern Scotland, 600 miles apart. The west coast of Britain is wetter and warmer, the east coast dryer and cooler. Also taking into consideration that further north temperatures will be lower. I live 14 miles west of Inverness which lies on the east coast. it is very difficult to explain the weather patterns here, not only in Scotland but all over Britain without having to write in length. I've attached the link below, that you can have a 'play with' but it's not totally accurate.
Maps of climate variables for previous months, seasons and years
www.metoffice.gov.uk
Even when I lived in England many years ago the weather was unpredictable. I can remember once in June when we experienced snow, wind, hailstones, fog and sunshine all in one day. The temperature at that time should have been approximately 20C/68F.
Seems warm enough for them. If they can grow them in Holland they should grow there.
Holland is further south than Britain, so it follows that it is warmer. I can't grow tomatoes outside here, summers are shorter and not as warm as in England.
@zigs knows more than I do about climate/weather in Britain so perhaps he may think of something to add. He often jokes about me living in the frozen north.
