1st step?
slash Army to 50,000, scrap all the ‘tradition’ that actually doesn’t date back more than a century, turn it into a proper military force, not a glorified boys club with huge numbers of troops dressed as the Ruritarian Palace Guard.
the 7th Loamshires will work just as well as the 7th Infantry Regiment, they will function just as well with a standard Army dress uniform sans ostrich feathers and scarlet tunics. The RAF and RN function perfectly well with that model.
RAF and Navy get the lions share of the budget - we’re an island, WWII ended 75 years ago - enough with the planning to fight wars on mainland Europe, leave that to the EUropeans.