Hi.
I think you mean SASE? SAE isn't stamped.
Anyways, did you try this one?
http://postcalc.usps.gov/ They assume your sending from the US, so you'd select UK. The date doesn't matter and neither will the flat rate service because it just predicts delivery. You select the shape. (I'm just assuming all here) I picked "large envelope" and entered "0.3" as the ounces. Don't check any of the boxes on the next step unless they apply to you.
Then when you get to the 3rd page, click "First-Class Mail® International Options" which will be the 3rd blue stripe.
If my calculations are right, you'll need 1.30 USD for stamps and the Liberty Bell forever are currently 0.44

So probably 3-4 stamps since it's overseas.
I explained all this in case you might want to use it in the future.
Hope I helped.
