Forum members in the Autographs for Sale/Exchange section sell them, you may also find some on eBay. If you know anyone in America, maybe ask them if they could help. I can't remember if you can buy them direct from USPS anymore, you used to be able to (over the phone, if you had a credit card).

Fantastic, where do i buy these stamps, is there a recommended seller?

Yes, if you want to purchase some stamps from a member here, you can do that. There's no such thing as an "airmail" stamp, they work the same way as ours do i.e. you'd just buy stamps to cover the value of the weight/size. For an A5 envelope with a letter, protecting card & a photo I use 3 "Forever Stamps". These currently cost $0.44 and work the same way as First class, i.e. if the cost goes up, you can still use the stamps (because there is no money value printed on them).

You can check the value of your letter by consulting the USPS website
http://www.usps.com/prices/first-class-mail-international-prices.htm

(Make sure take into account both size, weight, destination - UK is price group 5 - and whether it's a rigid envelope).

After looking through the forum i have noticed that people sell US stamps, is there an alternative way? possibly getting US air mail stamps and putting them on the returning envelope?

Thanks

Yes, probably 2-3 (depending on size & weight of your envelope). You dont stick them to the envelope or anything, just put them in with your letter & photos (if you're sending some). Maybe attach with a paperclip to the SAE.

Hello.
If i was to post a letter to someone in America, what would i need to purchase so i pay for the return postage?

http://www.royalmail.com/portal/rm/content1?catId=5200012&mediaId=27000662
An international reply coupon?

Thanks