Data point: Checked into Hilton London Kensington yesterday.
"No upgrades available."
Offered to pay for upsell to suite but stressed the price needed to be right and I wasn't prepared to pay the full difference given my existing upgrade entitlement.
Quoted 40 GBP per night from base room to suite, reduced from a typical 70-80 GBP difference (I checked at booking).
I accepted this, but I have half a mind to drop a note to Hilton corporate after this stay to ask whether denying upgrades then accepting money for them is against the terms of the programme!