Indeed though using a token number of Avios to leverage a corporate hotel rate is a very good idea, you have hit on one of the key restrictions. I seem to recall the reason being is that some of their hotels are booked by a back office via faxes, and rather than separate out the electronic bookings from the rest, they just came up with the blanket rule. On the whole it's best to avoid paying fully in Avios, the implied price is quite poor.

My tactic is to always have a bit of points lurking with IHG (Holiday Inn and what-nots). Their points plus money scheme is better than Hilton, particularly at the last minute, so you can generally stay somewhere for US$36 (£27) plus a stack of points. I'm not a fan of this chain at all, but I'm forced to concede they often come up trumps, and it's bookable via App.

[For Heathrow irrops, the Holiday Inn Ariel, at the end of the free bus zone at Harlington Corner, has "Executive Rooms" in this scheme and often availability when the Hiltons / Doubletree are on silly money. It's well placed for transfers between T5, T3 and LGW/Feltham]