The average UK size is a size 12 - 14, although today someone actually told be it's now gone up to a size 16. Now personally I don't think a size 16 is exactly a healthy size.
But a size 10 is deffinately not fat, a size 10 to 12 is classed as the perfect size! A woman needs a figure! A bloke would much prefer a woman with breasts, a waist, a bum and a nice pair of legs! No, I'm not saying flat chested girls, or really skinny girls, aren't attractive, but in honesty, I've never met a guy who has been attracted to a girl without a figure... a girl needs curves, and curves does NOT mean fat.
The media have made women/girls believe they should be stick thin, petite little figures! But 98% of those images we see are photo shopped to make the women smaller than they are, to smooth the edges and to give them perfect bodies! Bodies which aren't realistic.
No, I'm not encouraging obesity, but realistically as long as you can still see your own feet, so long as you get exercise and you don't get out of breath walking up and down the stairs, then don't worry too much.
As long as YOU are happy when you look in the mirror, and so long as YOU are healthy. Then don't worry.
But to answer your question, NO a UK size 10, or even a US size 10, is NOT fat.