Very nice. A superb example in beautiful condition. It's not actually "Pattern 1925", though, it's an early rendition of the post-WWI Mills "Web Equipment, Officers". This was the second version of this equipment, after the 1917 WWI issue. The sword frog is usually not there - I believe it was an extra cost option. The pistol case is missing, as you mentioned. It would originally have been sewn together with the cartridge pouch - if you look at the bottom of your pouch you can see where they have been separated. I've attached a photo of a 1922 example of the W.E.O. pistol case & cartridge pouch - yours would originally have looked like this.
After 1928, the style of the brace attachments changed from the round loop to a '37 style rectangular link. There is another set shown on Karkee Web - it doesn't have the sword frog but does have another optional extra, the blanket straps.
A bit more info on your W.E.O. setup. The quality of the photo below is not great, but the original was behind glass in the old pattern room at Leeds. It's taken from a 1919 Mills Catalogue of Officers' Equipment, and shows the basic set - there were many other options, such as your sword frog. The haversack shown in the photo was the Pattern 1919 type, but by 1927, when your set was made, it would have been identical to the Patt. '25 one - a haversack from one of those "Patt. '27" sets that you had a few years ago would be perfect. The braces, too - as you say they were identical in form to the Patt. '13/19/25/early '37 type. Pukkah W.E.O. braces were made in a fine, herringbone web to distinguish them from the ones for the oiks, but again a pair of your '27 braces would be an excellent match. The rucksack for the officer's set was ALMOST identical to the Patt. '19 one, or the one for the Belgian M1915 equipment. The only difference was on the flap - the naval version had an attached strap for the mess tins, whilst the officers' version had two short loops of 1-inch web, sewn horizontally one above the other. There were two main types of water bottle carrier. The type shown in the pics is the most common - in form it's identical to the naval Patt. '19 wbc (but not the infantry one, which was different), but again made in that herringbone officers' weave. The Canadian variation of the Patt. '37 wbc, the late style that has the snap on the front horizontal, is a good match here. The other type of wbc was a wide one for the India Pattern bottle - very much the same as the ones made for Sam Brown equipment, just in web instead of leather.
And finally, one other thing. We are very actively researching W.E.O. for a new section of Karkee Web. Can we have pics of your set for inclusion in that?