I cannot identify Keene & Son as British gunmakers, but J.W. Keen & Son were located at 61 Gloucester St. in London, and made P1853 Enfields for commercial sale. I have seen a few of these, marked KEEN/LONDON on the lockplate - one of these was carried by a soldier in the 64th Ill. Inf.

Value is dependent on many factors, including where the gun might be located or sold (the market), the NRA-standard condition, and proof of the provenance. If the gun has bona-fide Confederate markings, this adds substantially to the value.

Hope this helps,

Geoff Walden