Yes that's correct (it's spelt 'ullage').
However the definition of ullage is the amount by which a container falls short of being filled to it's capacity, but for pubs etc it's basically the amount that has to be chucked away.
The ullage is usually what is collected from the drip trays. Unfortunately the ullage has increased massively over recent years due to the use of branded glasses which don't have a 'fill' line, and so the drink is poured until the froth flows over the top of the glass into the drip tray.
That's more of an issue with lagers which are fizzier than other pump drinks.
You probably remember when all pint glasses were standard, clear items, with a line printed on the glass about half an inch from the top to show exactly where a pint measure of liquid was (necessary in the UK as by law, you are purchasing an exact measurement of alcohol so you can see that you're not being 'short measured'). The Use of these slightly oversized glasses meant you could get your exact pint, + a 'head' on the top, and there was very little wastage, which is something that publicans have been trying to get back into usage.
Due to the use of branded glasses without a 'fill line' it is now fairly commonplace for premises to try to get away with pouring you a pint with a big head on, claiming that they're allowed to go to 95% of a full pint, which is total tosh, you should ask them to fill it to the top, as, by law, they have to give you the pint you are purchasing (this dates back to the 'weights and measures' taxation which applies to all taxable goods sold in the uk by weight or liquid measurement, e.g. coal, cement, flour, petrol, booze etc etc).
