When I look around this small island, I see massive tracts of land entirely unpopulated by anyone.
Sadly by even most of Europe standards they dont even approach massive in proportion to the size of the country - forest areas need doubling at least
The UK isn't overcrowded, it's just appallingly badly distributed. Guess who wants to keep it that way?
Strongly Disagree viz over crowding - but agree with you on distribution
Current UK Population is 65 Million thats about 20 Million* more than it can feed and about 25 - 30 million too many in England. Theres also the issue that seems to be orgotten when people dicuss how many can we feed - Water- and we already have issues in summer in England - adding to the UK population is only going to make matters worse.
As to overcrowding
Further compounding matters is a need to increase forest cover - some of which can at least use marginal farm land and with effort we could re wood the Penines in parts (Thin soil thanks to deforestation complicates this).
Then theres the issue that building new homes further encroaches on farmland and or flood plains.
Ironically perhaps in light of my support o your remarks viz distribution, in order to sustain the population - we need it to be more concentrated (Better distrubeted accross Isle but denser smaller major Urban centres ) -Thus sprawling cities like london need to turn to high density urban estates using more low rises (2-4 storeys) rather than vast estates of terraced housing thus reducing the size of a city for a given population.
That would allow us for example to rewild tracts within the M25 (Actualy the ideal is urban Farm surround then wild - but It probably isnt possible to reclaim farmland not in short to medium term.
In short whilst you can argue open spaces = not over crowded - Others can equally argue that since those big open spaces equate to sustainibility (and also many arent suitable to build** on) then the country is unsustainable as such its over crowded.
*Its currently closer to 25 but a small change in what we farm and stop building on farmland
**On that note major Urban centres tend to be for obvious reasons adjacent to good farm land - logically the best place to build cities would be the highlands and Brecons - to make best use of land availible