This doesn't really seem relevant since no one is disputing whether the UK as a whole has enough space for more houses. The relevant comparisons are Paris vs London and then the various 2nd tier cities in the countries. Paris has a MUCH HIGHER population density than London, in part due to the housing mix, e.g. Parisian mansion blocks.

Prosperity comes from cities where densification is allowed (far too much of inner London is only 2-storeys) and being able to affordably put in supporting infrastructure.