We aren’t actually that high up the population density ratings though
Well, a lot of the UK is still green space and Wales and Scotland aren't densely populated. Scotland has 5 million people.
A main problem is while our population rises our infrastructure doesn't improve with it. You only need to look at the housing crisis and the pressure on the NHS/public transport etc, yet it's risen by over 10 million in 50 years, or nearly 30 million in 100 years. In Between the 30's and the 70's we built the infrastructure more as the population grow.
I think it's right that we want to protect our greenery, but there's a trade off in that population controlled is required.