I don't think Hornblower is 'Celtic' at all, depends what you mean by English and also what time period?
A lot of literature about Britain, not; Great Britain is focussed around king Arthur because it's one of our major legends/myths.
I quite like Bernard Cornwall's take on this. There are a couple of books which I don't have to hand and can't give titles to, that deal with Celtic culture in northern Scotland (Lord of the Isles, Brochs etc. Will try to find them and get back.
Try The Goddodin.
If we're on to original sources, then few things beat the original Cuchulain stories in the Irish Ultonian cycles. You can find them online for free here: