Yes, it's certainly the case that local effects can be greater but my point remains: the UK is not heavily urbanised and will not become heavily urbanised. We're not even talking about becoming as densely populated as Belgium or Holland. Of course, if you live in a city you'll be surrounded by city but that doesn't mean that there isn't green space out there and whether you've green space near you in the city is down to city planning not total population.
(Incidentally, what is that actually a map of?)
England as a single entity is more heavily populated than both Holland and Belgium though. Scorza has a good point about Scotland. Take away the two countries that are mostly uninhabited/uninhabitable and the country that will absorb most people is England, which is one of the most highly populated countries in the world, outside of small islands and city states.
Then there's the fact a "green space" is not really what we should be aiming for. Almost all of the U.K. is industrialized and even with that we can't provide anywhere near enough food for our population. There are pretty much no natural spaces (IMO far more important than so called green spaces - do road verges count in that?!) left in the UK, not even in Scotland and Wales. It's one of the reasons I ended up leaving. I just can't deal with only having the occasional "green space", rather than real places that you can actually get away from people and road noises and see endemic wildlife, rather than just a few rabbits, sheep and a fast declining population of songbirds.
Edit: unlike Scorza though I don't believe immigration is really making much of a difference to all this. We were like this way before the recent immigrations.
