The threaded Medium bases you use in the States are E26. E for Edison Screw, 26 for the width in mm. In the UK/Europe we use the E27, which strangely is just about the same diameter and thread pitch, but is a bit taller (one more thread). Usually US bulbs will fit UK sockets and vice versa, but not all of them. The threads on some E26s I have are not as deep so they tend to get stuck in some types of E27 socket.
Doug is right about the bulb shape. A actually means "Arbitrary" referring to the continuous curved shape that originally came about in the 1880s-90s when the glass was hand blown. UK household bulbs are usually "PS65" meaning Pear Shape, 65mm accross the widest part. Technically the P stands for Pear (shorter, fatter neck than an A lamp) and the S stands for "Straight", referring to the straight sided neck our bulbs usually used to have. These days they`re getting smaller and less defined but the PS name still sticks.
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