A few more suggestions. I should add a disclaimer to the effect that I have a dd who is only just 8, but she reads stuff that is aimed at older kids, so some of this might be suitable.
The Ingo series by Helen Dunmore (now up to 4 books: Ingo, The Tide Knot, The Deep, and The Crossing of Ingo. These are fab (and quite long, so will last a while).
Have you already read the Roman Mysteries series?
DD has just read I Coriander by Sally Gardner and loved that.
Also anything by Cornelia Funke has been a big success. Some are aimed at younger readers, but Inkheart/Inkspell might be OK for slightly older children.
You mentioned ponies, so you could try My Friend Flicka (and sequels) by Mary O'Hara. (DD has just read that, but got frustrated by the fact that the protagonist was a boy and that girls were portrayed as 'sissy'!) There is always Black Beauty as well [pack tissues...!].
DD has also recently enjoyed The Fire Thief, which is a retelling of the Prometheus myth.
The Britannia series looks interesting Marina. When you say that it's aimed at over-8s, is that because of the language or because of the content? When are you off to B-C? I am having withdrawal symptoms, as only going to Devon this year.