Checked day room rate for a random date in October, got GBP99.00, but only available between 10am to 4pm. I think this is too short.
A T5 Sofitel day room is about GBP135.00 and allows 8am to 6pm.
Both quotes are for members with no status. Early/late check-in may well be possible, but the published HGI times deter me personally.
It's good that there will be more options though.