Here's another approach that is short. The disadvantage is that you can only break the outer loop, but sometimes it's exactly what you want.

for a in range(10):
    for b in range(20):
        if something(a, b):
            break  # Break the inner loop...
    else:
        continue  # Continue if the inner loop wasn't broken.
    break  # Inner loop was broken, break the outer.

This uses the for / else construct explained at: Why does python use 'else' after for and while loops?

Key insight: It only seems as if the outer loop always breaks. But if the inner loop doesn't break, the outer loop won't either.

The continue statement is the magic here. It's in the for-else clause. By definition that happens if there's no inner break. In that situation continue neatly circumvents the outer break.

I would run the loop until ans is 'Q' like this:

ans = 'R'
while not ans == 'Q':
    print('Your score is so far ' + str(myScore) + '.')
    print("Would you like to roll or quit?")
    ans = input("Roll...")
    if ans == 'R':
        R = random.randint(1, 8)
        print("You rolled a " + str(R) + ".")
        myScore = R + myScore