The issue is this - the American military Reserve/NG is not very truthful, in my experience, with the time commitments. The marketing slogan is "a weekend a month, two weeks a summer" of time commitment, or something to that effect.
In reality, as a National Guard member you'll see commonly see month-long "activations" to go supplement active duty military training in Louisiana in February. You'll see 9 month deployments as R/NG are in fact part of the military and will deploy, and you'll go over there with a fraction of the training, funding and equipment but the same mission requirements as the funded, trained, and equipped active duty unit you replaced. This is somewhat akin to a FAANG hiring a suite of contractors and subbing them in to take over Google Search for a quarter or two. When your R/NG Soldier has domestic issues, you as the R/NG are on the hook to go help them quite literally. The list goes on... All for pretty bad pay, so-so healthcare, and a civilian employer who starts having an aneurism about the lack of managed expectations from you as it's not a weekend a month and some summer vacation time.
To bring this back to tech - the situation above is very well known, and it coincides with the military's efforts to heavily ramp up its technical expertise via "software factories," civilian corps, direct-commissions into elevated ranks at higher pay if you're a technical SME like the military does with civilian doctors, and reserve units focusing on cyber or software development.
Unless the military is able to fix that above, which anyone who has spent a shred of time in active duty military can confirm exists in one form or the other, the upside-down logic of R/NG service will keep good technical hires out... unless they really need the healthcare. The talented ex-military who go into tech stay out of R/NG efforts because of this, as serving actively would damage their career.
I hope it is different in the UK!
Edit - I will put a hopeful ending to this: the PLA's (CN mil) hackers go to "work" in an officer tower in a city, wearing civilian clothes, and (at the risk of speculation I don't enough about probably) work a 9-5 or if they're up late, it's for a mission. I say this with some background in the US version of ex-mil in tech: the moment the DoD and Federal agencies enable its technical hires to go to work in NY, Seattle, or SF in a fiber-equipped office building (or really just not rural West Virginia/North Carolina/Alabama/Mass), can meet at least 60% of private sector pay with some tax benefits perhaps, and gets sane about past civilian marijuana use re: getting a security clearance, they will have more technical patriots signing up than they can handle processing. It's really just this simple. The FBI was trying to hire talented CS students out of college with a great opportunity to go work in their brand new facility for tech that they built in.... Huntsville, Alabama.