Like many governments, the devil is in the details. A lot of what is included in this has already been announced or planned.
The actual new additional amount is considerably less.
£16 billion over four years is the equivilent of 64 new Type 31 frigates (at £250 million each). There should be plenty of money to build up a decent sized fleet to make a full carrier strike force in addition to the new cyber and space capabilities.
The ambition to become the second-largest defence spender in NATO is a way of building up diplomatic clout after Brexit.