We would call that keeper's tweed, but it's a slightly lighter weight than is usually seen on a riding coat. My own is 35 oz and it's definitely thornproof - I posted a photo of it here some time ago.It most certainly is a twill, yet I'd swear I've read it referred to as keepers tweed, though it's very different from what I think of as tweed. Seen some mercers selling it, and it's really heavy, around 20oz or so. Very tightly twisted extremely tough yarn is what does it evidently.
The coat shown is cut for riding, obviously, and as such would usually have a panel back, but for some reason does not.
Harry Hall has been well known here for decades but I had been under the impression the firm had for some time confined itself to the bottom end of the ready to wear equestrian market. Perhaps I was mistaken.
"Ratcatcher", by the way, simply means less formal hunting wear, as worn for autumn hunting ahead of the opening meet, and while a coat in keeper's tweed, worn with a coloured stock and brown boots, is the most common interpretation, a coat of almost any tweed might be worn. In an attempt to tone things down, some hunts in England now wear ratcatcher throughout the season.