Scrooge attended boarding school (he revisits it in his first visit with the Ghost of Christmas Past) so received pronunciation (sometimes called "public school pronunciation", or PSP) makes sense. As a filmmaker, received pronunciation would also suggest power and authority, suggesting Scrooge is "The Man" and so fit with his role.

British accents were a lot less homogeneous before broadcast media. There's a reason why RP is sometimes called BBC English.

It is common for biographies of 19thC worthies to mention regional accents. Tennison was noticeably Lincolnshire, a lot of people talked about Dickens as a cockney-flavoured speaker albeit not of the full lower class aitch-dropping variety.

TLDR Scrooge unlikely to speak RP. Actors very likely to do so.