History of the Great Britain Postage Rates

I'm now making a collection which I'll exhibit at the upcoming national exhibition.I'm now researching some postal rates on the covers and I found a website Letter Rates 1840-1968 of G.B. Unfortunately it does not show the international rate of the classic period, so I have to research more....

The shown is an entire letter to U.S.A. franking 1840 Pence Blue, horizontal strip of 4. I'd like to know if 8 pence is a right rate for this letter or not...