Keith Cochran's "Colt Peacemaker British Model" lists the following known caliber markings on "British" revolvers: ".476 CAL", ".476 CAL / E", ".476 CAL over .455 CAL", ".455 CAL". ".455 CAL / E", ".455 ELEY", ".450 ELEY", ".450 CAL /B", ".45 CAL / B". The "E" and "B" letter suffixes are generally regarded as signifying "Eley" and "British", respectively, although it has also been suggested that the latter could signify "Boxer". In any event, they are only seen on revolvers in "British" chamberings.
He does
not list .450 Eley Short, .450 Boxer Long or .450 Boxer Short as known caliber markings on revolvers.
(There are also a few observed examples caliber-marked ".380 ELEY", but those are irrelevant to this discussion.)
Cochran puts the total number of "British Model" revolvers somewhat higher than you have indicated ... arriving at a total of 4,738 Peacemakers, 215 Flattop/Flattop Long-grip revolvers, and 381 Bisleys (Target and Standard frames) for a grand total of 5,334 "British" chambered revolvers. Significantly more Model 1873 Colts were sold in Britain, but most of the rest were chambered in either .45 Colt or .44-40 (i.e. .44WCF) with records of six revolvers chambered in ".44 German" being shipped to the London Agency in 1882 and a single Flattop Long-grip model chambered in .22WRF (one of only two revolvers known to have been so chambered) being shipped to Purdey & sons in 1889.
I believe it is correct that the names .450 Eley, .450 Adams, .450 Boxer, etc. do in fact refer to the same cartridge, or at any rate virtually identical cartridges produced by different manufacturers which are so close in actual dimensions as to be effectively interchangeable. I base this both on the brief statement to this effect in the COTW entry for the ".450 Revolver" (i.e. "It is often listed as the .450 Short, .450 Adams and .450 Colt") as well as my own knowledge of the development of the common British "military" revolver cartridges.
You will note that I included the ".476" and ".455" cartridges in the above list ... and I have done so for a very good reason: the fact that the revolver cartridges variously referred to as .450, .476 and .455 caliber are in reality all the
same caliber - namely, .455! Look closely at this diagram compiled from a set of images in "The Webley Story", giving the case and bullet dimensions of all of the 19th century British service revolver cartridges -
Note that the outer case mouth diameter of
all of these cartridges was .477" (+/- .002") ... and the bullets were
all .455" in diameter. The differences in nomenclature reflect the lack at that time of standardized practices for referencing the caliber of firearms - some referring to the diameter of the case, others to the diameter of the bore at the bottom of the rifling grooves, others to the bore diameter at the top of the rifling lands, and so on ...
The first standard self-contained metallic revolver cartridge in British service was the .450, for which the Adams service revolvers were chambered, which is why it is referred to as the "Adams" cartridge in the above diagram. The original method of constructing the cartridge case - i.e. a separate base disk "riveted" to the case body by the primer - was the method developed and patented by Colonel Edward M. Boxer (Royal Artillery, Superintendent of the Royal Laboratory at Woolwich from 1855) ... thus the common reference to it as the ".450 Boxer" ... just as the similarly constructed cartridge for the Snider-Enfield rifle was referred to as ".577 Boxer".
Here is the original listing for the "Cartridge, Small-Arm, Ball, Breech-Loading, Boxer" for the Adams revolver (the Mark I version of which was a conversion or earlier percussion revolvers to metallic cartridge, soon followed by the Mark II and III versions which were designed and built as cartridge revolvers.) Note the reference to the bullet diameter being being ".455 inches" ...
Perhaps needless to say, the .450 cartridge case was eventually changed to one-piece drawn brass construction. To illustrate both the confusing nomenclature and the interchangeability of British service revolver cartridges ... when the so-called ".476" Enfield revolver was adopted to replace the Adams in 1880, its caliber was actually referred to in the List of Changes entry as "0.45 inches" and the entry also included the following statement:
"A special cartridge will be prepared for this pistol but, for the present, the service Adams' revolver ammunition will be used with it." Similarly, when the Mark I Webley service reolver was adopted in late 1887 to replace the Enfield revolver, it was used with the exixting "Enfield" cartridge for quite some time, as the "Cartridge, S.A., Ball, Pistol, Webley, Mark I" was not adopted until 1892 ....
Note: if you do not have access to a copy of Cochran's "Colt Peacemaker British Model", I could scan the pages covering caliber marks and cartridges for you ... to send that to you I'd probaly need your e-mail address, which you can hopefully provide via PM ...