Surely maintaining a well staffed, trained, disciplined, equipped and motivated military is something in the best interests of everyone?
From a purely military point of view we are honouring our commitments to NATO and have the capacity to support our allies and project force if required (particularly against the backdrop of the war in Ukraine, and any potential Taiwan shenanigans).
Of the money spent on the military a decent chunk supports R&D, hardware but also tactics which can subsequently be exported internationally. The same goes for training, which is world class (whether that's Sandhurst or Dartmouth, or us training Ukrainians).
You take a cross-section of British citizens and teach them a variety of highly useful and transferrable skills, all valuable whilst they are in the Forces and probably still beneficial to the country after they leave and get 'normal' jobs. The money invested, and spent on staffing, stays mainly within the British economy providing further economic stimulus.
Plus all of the soft power Hearts and Minds.