Capt Call, I made this list for you on Blue Book information for the London made Colt's.
These are the percussion guns known to have been assembled in Colt’s London factory.
English Hartford Dragoon, basically a 3rd Model Dragoon in .44 cal. 10% value $6,000.
1849 London Pocket Model, same as the 1849 Pocket Model in .31 cal. 10% value for Early Type (under SN: 1500 and with brass grip strap) $875. 10% value for Late Type (oval trigger guard and steel back strap) $625.
1851 Navy London Model in .36 cal. 10% value for Early First Model (below SN: 2000 with brass grip strap) $1,175. 10% value for Late Second Model (large round trigger guard and steel grip strap) $1,100
1861 Model Navy (London Marked) in .36 cal. 10% value $1,475.
1862 Police Model (London Marked) in .36 cal. 10% value $1,800.
1862 Pocket Model Navy (London Marked) in .36 cal. 10% value $2,375.
Colt’s London factory was in business from 1854 to 1864. The London guns were all marked on the barrel.
Because of your description of the revolver you saw, I only listed the lowest current market value of 10%.