There's plenty of land in the United Kingdom.
I would sincerely have to say.... ...utter bollix.
Compare to our nearest neighbour, Alb's favourite France, we have a similar population but around a 3rd of the land mass.
We need (undeveloped) land for a variety of reasons, not least of which to provide food, but also to provide drainage.
The solution is less people, not more housing.
Also:
there are areas of land that are unsuitable for housing, some of which have already been used for it. That's also true of farming. And any other purpose you can think of.
If a lack of land was really a problem, we would have to stop the growth of developments that inefficiently cram in lots of tiny semi or detached properties each with a minute garden. We'd be building vertically, which we've been doing in cities for decades for that very reason.
Approximately 1% of the UK's area is already covered by roads....
The UK hasn't been self-sufficient in food for 200 years, when the population was about a fifth of now.
A population reduction is very hard to achieve, and has all sorts of undesirable consequences. Just look at what China's one child per family policy did, and consider what is required to enforce it.
There are cultural differences to consider; you mentioned France, where property in rural areas is cheap and hard to sell. That's why many properties in north and western France were bought by British owners, and is a market that will be adversely affected by Brexit. Buying your own property is less of a priority for the French(most Europeans really) which is another important factor of prices.
Are there solutions to these problems? Sure, but none of them are palatable enough for us to accept being told to implement them. Look at the emotional reaction to being told to wear a mask in public for proof of that.