I actually thought of Meryl Streep as possibly portraying Aunt Letty. Though Mrs. LeFay does sound like an interesting choice. So maybe in the scene where Uncle Andrew tells Digory how he came to make the magic rings, a flashback of Mrs. Lefay while he is telling it in V.O. So that would be an interesting idea. 

Also, there's a certain balance in having both a male villain and a female villain (admittedly much stronger and scarier) who both, in different degrees, indulge in pride and arrogance and the desire to control and use others. It's good to show those failings (outright sins, really) aren't confined solely to either men or women. Having two female villains — Jadis and Aunt, er, Andrea — would diminish that strength of the story. Ironically, in the face of all the allegations that C.S. Lewis was sexist, it would potentially make the story sexist where it wasn't originally (because now both the main baddies would be women). {ym}:eyebrow:  

If Meryl Streep really is taking a role in this film — and like others, I'm a little more assured now that it's not Aslan, but I'm not completely confident there — I would also say she'd be ideal for either Aunt Letty or a flashback to Mrs Lefay.

Nor can she play Andrew  Ketterley, who fancies this amazing woman (Jadis)  and begins to think she might fall for him. 

Aunt Letty is much more likely. 

I wonder, what if Meryl Streep is up for the role of Uncle Andrew in the movie instead of Aslan?  The character would probably be renamed "Aunt Andrew" or something similar. Or, Aunt Letty could be given "Uncle Andrew's" role from the novel.  The movie could still be called the "Magician's Nephew" if Meryl Streep played Digory's michevious and devious aunt.  TBH, it would actually be a more meaty role for her acting talents than Aslan (who tends to be portrayed a bit more stoic).  

I really don't see a reason why the person who gives Digory and Polly rings has to be Digory's uncle instead of his Aunt.  It would feel less sacrilegious to some than the Aslan gender swap and it would give Meryl Streep a more showy role to show off her acting talents.

I also think he theory of her being Mrs. Lefay is interesting, although in that case it would more likely be a cameo than a supporting or main role.  

the only movie I can think of where she used a British accent was for ‘Iron Lady’ where she played Margaret Thatcher. 

see https://www.youtube.com/watch?v=xxcsGcV8s9g

She is famously a master of accents

I do have a question for other people who have seen more movies than me (which is about everybody), has Meryl Streep ever done a British accent? And if so, how'd she do?