The value of any "Antique" is always a very subjective opinion.
research what a similar item has sold for and you will have your answer.
If you're lucky about £400.00
$500
Library, internet, Standard Catalog of Firearms.
50-500 USD depending on specifics
500 to 1500
TOO COSTLY, RUINS THE VALUE, CHEAPER TO BUY A NEW ONE ABOUT $150
Really not possible to answer without gun in hand. Value of antique arms is driven by condition.
Around US$ 2,000 depending on its conservation status. Without DETAILED descriptions the range could be 100-10000 USD or more. While they may be old, a 1900 rifle is NOT legally an antique. US law has a specific legal definition of antique firearm, Unless they are muzzle loading guns, they must have been made in 1898 or earlier to be an antique. Values will be based on specific make, model, condition, and originality. without detailed information, we cannot even make an educated guess at value.
Value will be determined by how well it looks over a fireplace. Normally from $100 to $250.
Seldom beyond 100 USD or so
Isaac Hollis & Sons were noted British gunmakers from c.1860-1900. They made many top quality shotguns and rifles with many levels of ornamentation. This is one that should be individually hands-on appraised. sales@countrygunsmith.net
this is not exactly an antique or anything, mossberg made tons of these inexpensive shotguns-retail value:$75-$90