The gun room at William Evans is a great suggestion. Lots of very nice guns but the space is a bit cramped. I was there last May and the next day they had a sewer line break and the basement had several inches of sewerage - they had to close for a while to pump it out and deodorize!
North of Oxford in Banbury is William Powell - another very nice but small gun room. Lots of very good clothing. Powell used to make very good guns but now import Arrietas in the white and finish them. Not too fond of them.
