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
Hammerless
Information on this weapon can be found at any reputable gun dealer. And any bookstore will have catalogs on old or new firearms.
They probably denote what choke the barrel has.
The cartridges will pop out.
Rifle, handgun and shotgun can all have "long" barrels
I would look at the number of barrels. If there is more than one barrel, you know you have at least a double barrel shotgun. If there are 1 or less barrels, then you don't.
That would depend on the make and model of the shotgun in question,and the length of the barrel/barrels that came with the shotgun.
No published history on the company.
With the serial number that you provided,your Ithaca double barrel shotgun with the damascus barrels was produced in the year 1900.
100-300 USD
If you're lucky about £400.00
Try the gunbroker.com, or do a google search on the specific barrel you need. Mid West Guns also carries Browning Barrels.