Hi
Not sure I entirely agree though `apartment` is certainly more niche.
searching on google.co.uk
'london apartment` brings up 4,260,000 websites
'london flat` brings up 13,900,000 websites
so if you built and optimised a website around the the term `london apartment` it might be easier to get higher serps because there is less competition for that search term?
'liverpool apartment' brings up just 586,000 results to 1,710,000 for 'liverpool flat.' I'd say it would be quite easy to build a niche site around liverpool apartment and rank extremely well on google especially with that domain.