The Manton name was sufficiently recognized and valued by the market to constitute a valuable trademark. The trademark was continued into the 1900's even though the Manton's died in the early to mid 1800's. The Manton's, themselves, made no cartridge guns, only flint and percussion types.
This gun is an Anson & Deeley type boxlock with Webley & Scott Screw-Grip bolting (the word "Patent" on the lever is a dead give-away). W&S made Screw-Grips in boxlock and sidelock and in a wide range of original quality grades. Many Army & Navy and William Evans guns are on the Screw-Grip. Note that no model of the Screw-Grip was made to best work gun standards. I'd guess that this gun is from the late 1890's if the #7551 is the Manton serial as opposed to W&S serial (could be either way - look for the W&S S/N stamped on the barrel loop (forearm attach point)). For sure this gun was not built by the Manton's in 1821.
As to value in USA, the Brand Value of "Manton" marked cartridge guns is level three (BV3). This will be 1/2 the value of BV1 (Boss, H&H, Purdey, Woodward), and 2/3 the value of BV2 (17 well known gunmakers) for guns of similar original quality and similar current condition. The Original Quality grade appears to be about OQ6.5, between a second and third grade boxlock ejector ( I'm guessing this gun has ejectors based on model and what appears to be a split extractor stem in the first photo). Current condition would appear to be about level four, CC4; does "heavy use, but no abuse" describe the condition (although, the damaged screw slot in photo #1 casts some doubt).
BV3-OQ6.5-CC4 = $1500 for a 1890ish - 1960ish 12 bore SXS Brit/Continental gun of more or less usual models at USA retail. The gun is probably worth more in USA than the UK, but shipping may offset price difference on a relatively inexpensive gun (it cost no more to ship an expensive gun than an inexpensive gun, other than insurance).
I hope the foregoing is of some use, is understood to be based on assumptions from your description and photos, and might change significantly if the gun was actually in hand.