Thank you for the welcome Mervyn. Once the National Archives have digitised a file it usually can be purchased for a reasonable price from them.
Although getting copies of files that haven't been digitised yet is considerably more expensive.
Useful things that are digitised include WWI RAF officer service records, some WWI war diaries and most of the WWII RAF Operations Record Books.
Also most of the WWII War Cabinet Minutes can be had for free from TNA's website.
Lee