I never really got a "bump" when pregnant with my first (at the age of 18) I was 126 when I started and 152 when I had her, but i didn't get the classic baby bump, I pretty much gained weight all around, never really looked pregnant, just larger. So you can see if really varies from person to person.
i started to get my bump at 6 weeks believe it or not. it was my first pregnancy, i started out at about 135 lbs BUT i had twins.
There is really no way to tell you when you will begin to show. Some women show right away while it takes other awhile. It depends on a number of factors, including the position of your uterus. Ashelen has already said pretty much what I was going to so I won't go into repeating it, I'll just leave it as I agree with her :)
It actually has nothing to do with weight....some women show earlier or later no matter how much they weigh, lol. It has to do with the strength of your stomach muscles, the position, size, and shape of your uterus, the size of your baby (as well as whether this is your first baby or not...first pregnancies generally show later)...your levels of amniotic fluid and the size of your placenta....but at 19wks you should begin showing any time now. some women NEVER show even when they're tiny (I know that's not what you want to hear, lol) but it's very rare and most women begin to "pop" at around 20-25wks. So I would say any day now you'll start to see your bump. I don't know what it's like in the UK as far as maternity clothing stores, but sometimes you can find clothing that compliments your bump and shows it better.