What is planted in all the green fields in the south of England?

I just landed in Heathrow and the plane was in a lengthy hold over the south of London. It gave me a chance to look at the pretty green fields of that area. But it made me wonder : what is all that green? Just plain grass? Or are these fields used for crop cultivation (they looked too smooth for that). And what's the reason some of the fields have a group of trees right in the middle? Some of the fields are brown, is that an annual thing before onset of winter?

Thanks in advance for any helpful answers!