Despite the naming of Scintilla, it would be a good idea to use the correct terms or to simplify the process of choosing an encoding.
Stijn asked which value he/she should use for "character set" when using UTF-8 encoding. From Simon's explanation I think it doesn't matter. But why not grey out the option when using UTF-8?
The terms code page, encoding, character set are somehow mixed. I would prefer using the term "encoding" instead of "code page". The term "character set" is okay though it's unneccessary when using Unicode (UTF-7, -8, -16, -32, UCS-2 (which is indeed a restriction to the first 65000 characters)).
BTW:
There is still no option to set the encoding when saving a file or otherwise change encoding without changing the file's content.