The following photos and movie stills are contained in Leonard Mosley's book, "Battle of Britain" The Making of a Film. (Ballantine, 1969). Mosley, a well-established British journalist and war historian, was given the job of writing the "official book of the film." The idea for the film came from S. Benjamin Fisz, who flew Hurricanes with the Polish Air Force in WWII. He had previously produced the film The Heroes of Telemark in 1965, a story of the Norwegian resistance. For the "Battle of Britain", he was able to get Harry Saltzman to produce it. The Rank Organization initially came on board to provide funding, but this fell through, as did a deal with Paramount Studios. In the end, United Artists financed the film.
The world was scoured in 1966 looking for airworthy examples of fighters and bombers. While they initially believed only 6 flying Spitfires existed, they discovered that there actually 109! The film used 27, 12 of which were airworthy. Hurricanes were another story--only 6 were still flying in the world, and the film company managed to acquire three of them.
Finding Messerschmitts and Heinkel bombers was solved when General Adolf Galland was brought in as the German consultant. He was able to point the production company to Spain, where the air force included a number of Messerschmitts which were going to be put up for sale, along with Heinkel He111 bombers that were continuing in service with the Spanish Air Force. Both types of German aircraft had been made in Spain and equipped from new with Rolls-Royce Merlin engines. In 1966, the individual asking price for one of these Me-109's with less than 200 hours on the engine was just $6300, less than the propeller alone had cost originally. The film company bought all of the planes and parts, enough to make 28 aircraft, for an undisclosed amount.
As for the Heinkels, the film company approached the Spanish Air Ministry for permission to film the bombers in the air over their Spanish airdrome, in scenes with the British and German fighters. They offered to pay all expenses for the use of the planes, fuel, and wages of the pilots, crew and ground staff. The Spanish government generously waived the cost, asking only that the company consider making a cash donation to the orphan school run by the Spanish Air Force. The film company later estimated that this gesture had saved them $420,000. Their donation to the school? $1400. Shameful.
All aerial filming was done from a specially modified B-25 Mitchell christened "The Psychedelic Monster" because of its outrageous paint scheme (this was the Sixties, remember). Apparently, it was flown back to the U.S. where it changed hands several times and was eventually renamed "Lucky Lady." It now sits effectively derelict outside of an airport at Franklin, Virginia, grounded as much due to legal as mechanical problems.
Although imdb claims the film lost $10 million worldwide, I think it's a great flick. It certainly couldn't be made today. The book is also well worth the time to read it.