Hi Mara,
The GB stamps are:
1 shilling SG150/Sc64 (most expensive plate number!)
3d SG144 /Sc61
Are you talking about the red cancellation on the shilling stamp? It looks like it has a crown at the top so maybe a registration marking? What year is the cover? From that we can find out what the rate was to the USA. The other red stamp looks like New York so maybe an arrival stamp?
Hope this helps
