Thank you BotB - very helpful advice! The Superior (lowest level) room was £ 243; Classic £ 324 and Luxury (which is what we wanted) was £ 294 (surprised that it was less expensive than the smaller Classic). So I figured that £ 294 for a Luxury room was good and I went ahead and booked it.
I am really looking forward to this trip; I'm the only one in our family who has never been to London!