I'm assuming you are going to have it sent to the US?
It's a flat item, so it would be essentially a large letter I think. Postage is done on weight and size and tracking is not automatic, it is an extra. That all said, for just an autographed card or photo, I imagine the weight and such won't make it too expensive to ship.
THe one concern I have with your question though is how you are going to send an envelope with Royal Mail postage on it to the UK as an SAE type thing. Are you planning on doing the transaction online? If so you will need to weigh and measure everything to get the amount right...but I don't know what Royal Mail's policy is on buying UK postage online with a card/account in a different currency.
I would suggest finding someone in the UK who can help you by being a middleman in the transaction. I think that will be your easiest option. I'd volunteer but I'm back and forth between 2 locations and one address the postal system is not totally secure while the other would involve burdening my parents, so I'm really not much use in that regard.
In terms of your original question, I went through the RM site's calculator and although I had to guess the weight of the item, I think your option is Air Mail - Large Letter and then either tracked or not tracked (the price is significantly different, from £2.50 for a really slow sea mail to £8.35 for a tracked large letter.)
It might be a smaller letter post item, but a lot of things are large letter these days, so I would work on the assumption that if it is any different in size from a normal letter, including making the envelope fatter than a folded sheet, it will be large letter.
I'm gonna assume you already tried this, but the link for the above price finder is
http://www.royalmail.com/price-finderIf it's any other location than the US the same rules apply but price might be different. Most post from the UK out is zoned but it doesn't always follow that the price of something within the same country zone is the same for all rates...