Hi George,
I think I can add something to your post, at least from an Australian perspective.
Britain, and by design, the Commonwealth also had the 15-pdr QF gun, as well as the 13 and 18 pdrs.
The 18/25 pdr was a conversion of the later models of 18-pdr: the MkIV with box trail, (in many ways similar to the later 25-pdr trail), and the 18-pdr Mk.V with the split trail, the nomenclature of the converted gun says it all:'Ordnance QF 25-pdr Mk.1 on Carriage 18pdr Mk.V(P)'. There were also a small number of 18-pdr MkIII carriages equipped with the 25-pdr ordnance, but these were only in UK service, I think.
Australia had a number of Mk.III and IV 18-pdr guns on the inventory prior to WW2, on various carriages, including the Mk.IV(P)(Aust) - the pneumatic conversion unique to Australia. As far as I can tell, there were very few MkV 18-pdrs in Australia at that time.
Conversion to 'mechanised transport' was even slower in Australia, with an interim period in the mid-1930s where pneumatic-tyred trailers (the Wilton Trailers) were developed to carry the 18-pdrs and limbers around, rather than conversion direct to pneumatic wheels (too expensive!). This overlapped with a period where tractors - little Howard wheeled farm tractors - were introduced as the 'gun tractor': they were as slow as horses, so didn't 'stress' the carriages!
In the early stages of WW2, after the 2nd AIF started arriving in the Middle East, they received a small number of split trail 18/25pdr Mk.V(P)s, with at least 8 issued to the 2/7th Field Regt, and I have an image of the 2/4th Fd Regt training with them in the Middle East in 1940-41, prior to their requippment with the 25-pdr.
I seem to remember the remains of a Mk.V(P) 18/25pdr turning up some years ago in Australia, so there must have been a few there at least, but I've not yet come across any official source that lists them.
Regards
Mike C