What kind of beers are to be served lukewarm?

English ale is served lukewarm while lagers are usually chilled.

Why? Mainly because ale yeast works best at room tempertures while lager yeasts like the cold. In other words, they keep working in the bottle and create a certain amount of carbonation from the primed sugar.

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I'm not sure 'lukewarm' is the correct description of the temperature ales should be served at. Lukewarm seems to me to imply at least some degree of deliberate warming, or some cooling from a higher temperature, so that it is somewhat above room temperature.
Like red wine, ale should be drunk at room temperature while white wines and lagers should be chilled.

British real ale should not be chilled like lager - the best temperature is a cool room temperature. In a hot summer, a degree of cooling is allowable to achieve this, but a good ale can be drunk at most British summer temperatures without this. Keg beer will come cooled, and if you have to drink it this is probably to the good. (Imagine drinking room temperature Coke as opposed to chilled. Get the idea?) Americans, being used to big brewery lager, get brain-washed into thinking that ice-cold is the only way to serve beer. In the case of most American lager, this is correct procedure. In the case of real ale, it spoils the taste.