Like edramshaw said, cask real ales are generally have a low level of carbonation, and the cask needs no pressure applied- the beer engine is basically a manual pump. The beer engine is often used to push the beer through a sparkler which whips air into the beer, giving it a dense, creamy head. Sparklers aren't found everywhere in Britain though.
One small point is that to prevent the beer spoiling if not consumed fast enough many pubs use cask breathers, which don't 'push' the beer, simply replace the beer in the cask with CO2 at atmospheric pressure 'on demand' (like a scuba diving regulator). CAMRA believes that cask breathers are one step down the slippery slope to ice cold mass produced beer and disapproves of them, but in reality all that is lost is the changing character of the beer.
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