The relevant legislation dealing with copyright in a UK context is the Copyright, Designs and Patents Act of 1988
(CDPA). Many of the statutory exceptions contained within that Act that we could usually call upon for educational purposes have a 'non-commercial' clause, which means that they can't be relied upon when including third party copyright material in courses of a commercial nature. This, however, is not true of Section 30 of the CDPA, which deals with criticism, review and quotation.
This exception will allow you to use an extract of a copyright work for the purposes of criticism and review or quotation. It relies upon the concept of "fair dealing", which means that you must:
provide sufficient acknowledgement for the published work you are commenting on or quoting, including its creator;
only use as much of it as is necessary for your specific purpose; and
make sure your use doesn't undermine the copyright owner's ability to exploit the work; always consider the financial impact of your actions. If what you intend to do will significantly damage the copyright holder's sales, then it is unlikely to be considered fair.