I am looking some something that will let me produce a UK profit & loss and balance sheet (initially, perhaps more later).

The thing is that I do to want to enter data manually for each transaction. I want to do it from an app.

If there any way add data from my app, maybe by an HTTP API?

Failing that, is the save file format published, or easy to reverse engineer? If so, my app could just write a save file and I could open FontAccounting to generate reports and print, etc