No, it won't. You can send mail out of your country with your stamps, but if the person you are sending to is sending out from the USA or UK, they need to have their countries postage on it.
If you can't get the Reply Coupons, the best thing to do is just send a SAE without any postage, and maybe include the equivalent of $2 in your currency so they can use their own postage (and you are still being considerate to pay for it). If you can go to a bank and exchange some money and send US dollars or UK pounds, that would be even easier on the person you are sending to.
You may also want to, though it's not really necessary, apologize in your request letter that you were not able to get postage or reply coupons and note that you've enclosed some money for postage.
Good luck!
