Earlier quoted context omitted.
These buy-to-leave flats are generally newly built (and were bought off plan), in high rise buildings with concierge / security, so there are no worries about heating, burglary or squatting... I'd guess the price of these is around £350k - £1.5m (for a reasonable 1 - 3 bed flat in a new build), which is well within the reach of well off foreign middle class investors.
It is within reach, but it would be financially wildly imprudent to forgo the £1000 - £3000 monthly rental income that such properties would fetch. Bear in mind that you will have to pay council tax and other fixed costs anyway. It would be a desperately terrible investment, much worse than other, more liquid assets. The global middle class cannot afford such investments. Maybe members of the Saudi royal family, but…
Incidentally, I just had lunch with an architect and a guy who runs a building company, both London-based. I asked them about this. They both agreed that no sane investor would leave flats bought as investments empty. Investment means basically having tenants. The agreed that there is a media-driven moral panic about this. The architect said he had one client who had a 12 bed-room house in central London that was empty for most of the year ... that customer was a member of Kuwait royal family.