This article is from the Real Ale FAQ, by with numerous contributions by Brett Laniosh others.
A: Yes at a last resort. If you are served bitter instead of mild or
vinegar instead of beer then make your complaint known. You have a right
to get what you ask for, and for it to be of reasonable quality. (By the
same token the publican expects you to be reasonable and know what you
want.) If the beer isn't what you're used to then that is the luck of the
draw. Real ale is never the same two days running. Regardless of "Jim is
drinking it so it must be OK" etc. etc. you can insist on either getting a
decent pint or your money back the choice is yours (see "How can I tell if
my beer is in good condition" below). If you get snotty responses to your
complaint don't go there again and tell the rest of the world not to go
there as well.