Ok, so I've looked at the USPS website and I don't think you need a standard International stamp, but may actually need two of them - IF your envelope is going to be not over one ounce. I believe the UK is in the 3-5 zone price range. If your envelope will be less than one ounce, that's the equivalent of two international letter "forever" stamps. I can send you those. If it's going to be more than an ounce but less than 2, you'll need three stamps. I have three stamps available - but one will need to go on the envelope to get the other two to you.
If you want the two I will send them, but I'm not sure it's going to cover the cost for a C4 envelope (as I don't know what one weighs). I don't know if they'd deliver it postage-due or return it to the sender if the postage is inadequate. Let me know if you still want me to pop the two I can send into the mail.
A standard sized envelope, international first-class mail stamp is $1.45 for under one ounce to zones 3-5
First-Class Mail International Retail Large Envelopes (Flats)
Weight Not Over (oz.)
Price Groups
oz 1Canada 2 Mexico 3–5 6–9
1 $2.90 $2.90 $2.90 $2.90
2 3.15 3.74 4.06 4.00
3 3.42 4.58 5.23 5.11
4 3.66 5.44 6.43 6.22
5 3.93 6.29 7.60 7.33
6 4.19 7.13 8.78 8.46
7 4.46 8.00 9.96 9.56
8 4.72 8.84 11.13 10.67
12 6.03 10.67 13.50 12.98
15.994 7.33 12.51 15.86 15.27
