I know families who own 'buy to leave' properties in Kensington, because I teach in a posh school in the ME, with many 'super rich' families who are looking to park their money somewhere stable (more so than their country of origin - there have been Interesting Times here in recent years). The political families in particular have always got one eye on the emergency exit signs...
Are they using that property, right now? Generally not. They visit London & stay in ritzy hotels, or visit family who are actually settled here. As university age dc set off for London they are bought practical flats, not installed in the family's huge decaying Miss Havisham style mansion.
Would they take kindly to their property being requisitioned, even temporarily? Fuck, no. They're from a country with huge inequality & are generally much bemused by any notion of social empathy. They bung their servants bonuses & do charity events - they aren't awful people - but any idea that it's unfair that some are living in mansions, some are living in holes is pretty alien.
How would they react to their empty London houses being requisitioned for the Grenfell victims? Well, it wouldn't hurt them, if it was temporary, compensated & they were cleaned up & refurbished as necessary afterwards, but they certainly wouldn't be happy.
The question is: is the UK wanting to be held to ransom by these guys? Is it worth the candle of making use of this property, through existing legal channels, & then facing repercussions in terms of their subsequently investing elsewhere?
I'd argue London eventually would be cheaper, healthier & generally better off without these zombie mansions being used to park large sums long term for private & non UK gain, but whether it's worth the interim legal & political shit storm I just don't know.
But I don't think it's an idea to dismiss out of hand.