Elizabeth McGovern (Downton Abbey 2013) as Leia:

Toshiro Mifune (Red Beard 1965) as Ben:

Prince (Under a Cherry Moon 1986) as Lando:

Oliver Reed (Sitting Target 1972) as Han:

Michael J. Fox (Secret of My Success 1987) as Luke:

and Michael J. Fox (Teen Wolf 1985) as Chewbacca.

Peter Wyngarde (Flash Gordon 1980) as Vader:

Can anybody else think of better?

Oliver Reed would have been fantastic as Han, I can totally imagine him delivering the "Kid, I've flown from one side of this galaxy..." line.