Britain is spending £54.2bn on defence this year.
GDP in 2023 was £2,690bn, so 2.5% is £67.25bn
So he's suggesting an annual boost of £13bn per annum.
The green investment pledge he ditched because it would be "too expensive" was going to be worth £28bn per annum.
Conclusion: He's specifically redirected a full half of the budget designed to make UK industry and utilities ready for the inevitable need to ditch fossil fuel dependency towards a fictional scenario in which Britain faces invasion (in reality a scenario where Britain's ruling class will be trying to keep up with the Joneses in a pointless willy-waving compeition).