The typical card has 16 numbers American Express has 15.
Credit card ID numbers have three (Discover, Visa, MasterCard) or four (American Express) digits.
Amex card has 15 digits.
There are several American express cards available. There is a corporate card, a gold card, a platinum card and a treasure card.
Here is 31 of them I know for sure American Express® Preferred Rewards Gold Card American Express® Preferred Rewards Green Card Bank of America Accelerated Rewards™ American Express® Card Blue Cash® for Business Credit Card Blue Cash® from American Express® Blue for Business® from American Express® Blue from American Express® Blue Sky® from American Express Business Green Rewards Card from American Express Clear from American Express® Gold Delta SkyMiles® Business Credit Card Gold Delta SkyMiles® Credit Card Hilton HHonors® Platinum Credit Card IN:CHICAGOSM from American Express IN:LA® from American Express IN:NYC® from American Express JetBlue Card from American Express® JetBlue® Business Card from American Express® One from American Express® Platinum Business Credit Card from American Express® Platinum Business FreedomPassSM from American Express® SimplyCash® Business Card from American Express® Starwood Preferred Guest® Business Credit Card Starwood Preferred Guest® Credit Card The Gold Card from American Express OPEN (SM) Bank of America® Accelerated Cash RewardsTM American Express® Card Platinum Card® Platinum Delta SkyMiles® Credit Card True Earnings® Card from Costco and American Express TrueEarnings® Business Card from Costco and American Express There is also the rumored American Express Black card http://www.creditimes.com/american-express.shtml
Many different companies offer the American Express card service. Companies such as Delta accept the American Express card and offer rewards for continued used with discounts on tickets.
American Express currently has 5 different types of cards. They have 6 cards if you count Premier Rewards Gold Card as a different card than the standard American Express Gold Card.
American Express has their own UK division. Because of this, your American Express card should be readily accepted in the UK.