The Spitfire was using "continuous climb" rating, Usually a 30 min to 1 hour rating, For the Merlin 45 this was 1200hp at 16,000ft and a bit less below that as the throttle had to be partially shut at lower altitudes to keep from over boosting.
Military rating is an American rating and was usually comparable to take-off power. For a good part of the war it was a 5 minute rating but
some engines had it raised to 15 minutes part way through the war.
Military power was also the power the engine made at it's best altitude with the throttle wide open. Any higher and the thinner air caused the power to drop. Any lower and the throttle had to be closed to prevent over boosting and damage to the engine.
The US of Military power did not require any extra maintenance procedures. WEP came into use (American) in late 1942 and 43 (depending on engine) and was achieved by opening the throttle below the best altitude (or Full Throttle Height /FTL ) or critical altitude. However the throttles were often fitted with tell tales (wires) which when broken signaled ground crew to perform extra maintenance checks. ALL use of WEP was to be noted in long books to determine adjustments to the engines overhaul life.
The Spitfire V was allowed to use 3000rpm and 12lbs boost in combat
normal. at some point after this test. as a 5 minute rating and was finally allowed to use 3000rpm and 16lbs of boost (for 3 minutes) as the British equivalent of WEP. However the engine could only hold 16lbs to 11,000ft.
horsepower was 1515 though.
You might want to look at this test:
Spitfire Mk V AA.878 Report
and please note it was done about 2 months before the Corsair first saw combat.