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For fuel consumption do not worry the Spitfire can fly a long time if you manage your rpm's and lean mixture, I think two hours is possible. I never tried! There are 85 gallons of fuel in a full Spitfire MKI or II. But when you reach 5 gallons look for a place to land. One last remark most of the time in the Battle over Britain like the tittle says it you are fighting over Britain your country so no problem...
I did have a look in one of my books and at 14.000 feet with 1700 rpm and 150 mph a Spitfire MKI does have a consumption of 25 gallons per hours so could keep in the air for at least 3 hours.