According to Olympia London
website, there will be 2 onsite hotels, one is now announced the Hyatt Regency, I wonder which brand would be chosen for the boutique hotel. [Edit: the boutique hotel will be CitizenM]
This video shows the locations of the 2 hotels, seems like Hyatt Regency will be located in the additional building facing Kensington High Street and the boutique hotel will be in the carpark building. [Edit: It's the other way around, CitizenM in the extension above National Hall facing Kensington High St, Hyatt Regency in the car park building]
The impressive highrise in the back, unfortunately, is not the hotel.