I have no idea what St. George's cross has to to with any sort of mathematical proof. I simply don't know what you are talking about or what you want.
As to the question of why the product of two negative numbers is positive, look at it this way:
Mechanization of the operation of multiplication on the number line involves taking a number of steps equal to one of the multiplicands and each step being equal in length to the other multiplicand. For example, if you multiply 2 times 3, you could either take 3 steps each of which is 2 units long, or you could take 2 steps each of which is 3 units long and you would arrive at 6 either way.
Now let's add to that definition the idea of signed numbers. Let's adopt the convention that the sign on the first multiplicand tells you which way to face at the start -- that is, if the sign on the first number is positive, you would face to the right toward the positive numbers, and if negative you would face the other way. Further, let the sign on the second number be the indicator of whether you walk forward (in the direction you are facing) or backward (opposite of the direction you are facing).
Now, let's go back to the example of 2 times 3. Face right (toward the positive) and walk forward -- positive 6.
-2 times +3: Face left (toward the negative) and walk forward -- minus 6.
+2 times -3: Face right (toward the positive) but walk backwards (away from the positive) -- again, minus 6.
-2 times -3: Face left (toward the negative) and walk backwards (away from the negative; toward the positive) -- positive 6.
Hope that helps. Remember, two wrongs do not make a right, but two negatives do make a positive.
John