I understand that this might seem exorbitant but Jim and I no longer are middle-aged and we fly into London from San Francisco -- door to door that is about some 16 to 17 actual hours and assorted time changes inducing severe jet lag on our elderly bodies. So, hell yes I am willing to pay big bucks for early check-in.
As indicated above, I have some experience of the best London hotels with varing degrees of satisfaction over the last 25 years. Circumstances change over time yet it has been my experience to be delighted by The Savoy above others. Your mileage may vary, of course. And, that is what makes London such a great hotel destination, second to Paris ... but my travels are limited compared with most here.