You are correct that your New Service was made originally in .455 Webley, which Colt, for some unknown reason, marked as .455 Eley. (Some have speculated that Colt didn't want to put a rival revolver maker's name on the gun, but Eley was OK since Eley only made ammo, not guns. Like I said, speculation.)
The gun was taken into British service and presumably used in WWI and perhaps in WWII also. The crossed pennants are the British military proof/acceptance mark. There should also be a broad arrow property mark if it hasn't been marked out or a facing one stamped to make what looks like an asterisk.
After the gun was sold out of military stores after WWII, it underwent British commercial proof, part of which was the marking "NOT ENGLISH MAKE" which was used from 1925 to 1955 on foreign made guns.
Since it was usual for guns bearing the British military proof not to be re-proved under the commercial proof law unless altered, it is very likely that the conversion to .45 Colt was done in England, though many of those guns were converted here after import. If you describe or picture the actual proof mark, the case length will tell which.
Since the .45 Colt rim is thicker than that of the .455, the chambers could have been counter-sunk (the most common practice) or the rear of the cylinder turned down slightly.
Your gun, if in good condition, should be fine for any reasonable .45 Colt load or reload. But remember that the often wild overloads made up in that caliber should NOT be used in that old Colt.
Jim