Depends where you get your idea of it being heavily populated from really. Some of it is. Lots of it really isn't.Never ceases to amaze me that for such a heavily populated island that doesn't look that big on the globe the UK has such a large amount of wild and open land.
London is heavily with a population of over 7 million residents in an area of just 600 square miles
But head North to my country (Scotland) and it has a population of just 5 million but our land spans 30000 square miles. and 3.5 million of those live in the central belt (the area around and between Glasgow and Edinburgh (our two largest cities. Just 45 miles apart). meaning on the whole Scotland is actually quite sparcely populated
To give you an American perspective New York city has around 7 million population in an area spanning just 300 square miles