Not a great deal left in central London. I hear Holland and Holland now has just a room above the Beretta showrooms just up the road from William Evans in St. James’s.
William Evans also has showrooms at Bisley Camp, which also had Fultons (mainly rifles) and King’s Shilling Antiques.
If you have time to commute in Sussex there is J. Roberts and Son at West Chiltington, F.A. Anderson at East Grinstead.
In Kent Chris Potter Guns at Tunbridge Wells.
Use Google for directions, idea of stock carried and opening hours,