Possibly a bit late for this, but I just ran the Cancer Research 10k at Loseley, and it was far, far from flat. Assuming the same course (and there aren't a lot of options), then be prepared for hills - the CR10k course had 3 (1 long and steep, plus 2 other shorter but steeper). Having said all of that, for each of the ups, there's a down :-)
It is a wonderful place too.