I've done dayrooms at the Yotel and Hilton, both at T4. Hilton was lovely, it's nice and modern, could've stayed there much longer, and wasn't hugely expensive. Only £70 or so, and that was a walk-up rate.

The Yotel was an interesting experience, just wanted to have "done" it, but not one I'd care to repeat. Just too small and claustrophobic for me.