Upvote:5
Depends on the exact weight - a half-ounce letter would be 2 1/2d, a one-ounce letter 4d. (Postcards were 1d until 1921).
There were standard definitive stamps for 2 1/2d and 4d (blue and grey respectively) so it is likely these would have been used, but a combination of smaller ones might have been used if they were to hand.