I think IHG's footprint in London is (ironically for a British company) well behind Marriott's and Hilton's. I think Kimpton is also an option for "something a little less than IC". I also think Hilton and Marriott have more options in NY and Vegas.

I'm extremely agnostic about chains because credit card elite status neatly fills the bucket for me (Hilton Diamond, Marriott Platinum, IHG Platinum, some legacy Hyatt Explorist from MGM crossmatching that will become Discoverist next year). I don't travel enough to hit an organic status without having to compromise loyalty to my wallet with loyalty to a chain. IHG HIXes are fine by me a lot of the time (I'm staying at one on points in SF Union Square later this year where I, like you, just want a clean room with bed and a shower), but so are Hyatt Places/Houses, Marriott Courtyards, HGI/Hampton Inns.