I have been studying this issue of British guns for authenticity as I am also bldg. a SYW British contingent in 28mm. I have come to the following tentative conclusions. Based on the sketchy description of the British "light" or "field" carriages in John Muller's _Treatise on Artillery_ of 1757, the Elite Miniatures carriages look the most accurate for the SYW. The field carriages would have a set of "lockers" or "boxes" set over the axle tree on either side of the cheek pieces. However, Elite's 6-pdr. and 12-pdr. carriages are both too large and off-scale for 28mm, even for "heroic" 28mm. I use Muller as a source because he was a professor at the Artillery Academy at Woolwich, even if he has to be used with some caution, as he sometimes ventures off like a good professor to lecture his readers on what "ought" to be from time to time!
The Elite carriages are "wrong" even if artistically "close" because as one is a 6-pdr. gun carriage and one a 12-pdr. carriage, the models are the same size at 60mm long -- this is wrong. Carriages were built to a mathematical set of formulas based upon "parts" (or fractions) and multiples of the caliber of the gun being carried. Thus the side planks or cheeks of the two guns' carriages should have started from a base of 7 & 1/2 feet and added either 10 or 12 diameters (i.e. width) of the shot, depending on if a newer or older style gun was being carried. This would result in a "plank" of the cheek of the carriage being 12 feet long for a 12# gun, and 11 feet for a 6# gun (Muller, 1757; 164). Admittedly, the difference amounts to about 5mm or 6mm in a gun scaled for 28mm figures, but that is just about noticeable. I would add that the characteristic boxes over the axle trees on British guns were NOT indicated on these travelling carriages.
The "lockers" were described as being present on the "field" carriages for the new lighter guns that came into use in the SYW. However, these carriages were built to a slightly amended formula since for field use they needed to be lighter than those intended for the "battering" or position pieces. These carriages would result in the carriage being 9 feet long for a 12# gun, and 8 & 4/5 feet for a 6# gun (Muller, 1757; 182). THESE carriages would thus be almost identical, especially in 28mm scale, but notice that at 3 feet shorter than the travelling carriages, they would be almost 15 or 18mm shorter!
At 60mm the Elite carriages are therefore about 12 feet long in 28mm scale. That would be fine for the older guns' "travelling carriages" except that those carriages ought not to have the "lockers." Eureka's 6-pdr. carriages are next-best. They have the side "lockers" cast on, but lack the boxes that are supposed to slide on over top of them. However, at 54mm long their carriages are about 11 scale feet, so again too long for the "field pieces." Foundry's carriages at 48mm are closest in scale, but are the wrong gun carriages. They have the "side boxes" adopted to replace the "lockers" in about 1773-1776 (characteristic of British guns in the AmRev), and so are about the right length for field pieces at about 9-feet, but are historically inaccurate. Old Glory 6-pdrs. at 50mm long are about right in length -- their side "lockers" are present if a bit "weedy," and like Eureka's carriages, their top boxes are missing.
To solve this one is left either using inaccurate guns (Foundry), or constrained to having only 6-pdrs. in your contingent (Old Glory), or using 6mm as 1 foot not 5mm; allowing "heroic" 28mm to equal 1/50th scale not the more traditional 1/64th, which would allow integrating Eureka guns
It doesn't solve the problem of using the Elite guns as the carriages are still too large/long for "field" carriages, and artistically wrong for "travelling" carriages
I am however leaning in this direction. It would mean my troops were all nicknamed "Shorty" as they would barely top 5-feet in scale height. However, I could then use the Old Glory guns as 3-pdrs. I could use the Eureka guns as 6-pdrs, and also as "medium" 12-pdrs., as at Minden (with a thicker and slightly longer barrel). I would have to "hold my nose" and ignore the side "lockers" but I could also use the Elite guns as "heavy" 6-pdrs. and "heavy" 12-pdrs. on "travelling" carriages. Happy to entertain counter-arguments! Best wishes to all!