Book now. Everything is super expensive and getting more so by the day. You're looking at $400-500/night for most places in July and August.

The best values are east of "the city" For Hyatt properties, that would be the Hyatt Place and the Andaz (for some dates)

The downside is that most tourist sites (other than the Tower) are in the West End. So you might be riding the Underground a bit if you stay at one of these properties.